Since you didn't mention it was only characters in BtVS, i'm gonna count Angel ones too 😛
BTVS main characters(require at least three seasons of regular appearances):
Buffy Summers:
Favorite: Season five. This was the most touching Buffy got for me. She had to go through so many things, and for the most part remained who she was since the beginning. She shined at multiple times, but for me the best moments were when she did her speech to the Council and when she kissed Spike for what he had done 🙂
Least Favorite: Season six. A lot of peoples say that season is depressive and sad. Well I agree to a point. But what bothered me with Buffy in that season is how she barely looked like the girl we got to know from the previous seasons. The only few times I recognized her were like with Xander before she got shot, or with Spike at the wedding. Seeing her sit on her bed with a stake and a cross afraid of giving in to Spike was pretty disturbing for me. Buffy was stronger than that, I thought.
Xander Harris:
Favorite: This is pretty hard to choose, because for me Xander didn't really shine through a whole season. He shined at particular moments through the SERIES. But since I have to pick, it's gonna be season three. Outside the obvious 'The Zeppo' which I think is his best episode after 'Grave', I really liked him for doing what he does best, which is to be best friend for Buffy. And before you go and tell me he was a jerk to her multiple times in that season, let me tell you that sometimes that's what a best friend do, tell us things we don't wanna hear. Because we have to be put down to earth about some things. And who else but our closest friends to do that? We put so much faith in them, their opinions and feelings matter much more to us than if some stranger told us the same things. I also liked what he did for Cordy at the end of the season. Probably the nicest thing he did for her, and it was when they not even together anymore.
Least favorite: Season seven(you gonna see this number a lot, lol). It's pretty easy to pick this one. In fact thanks to it I don't have a hard choice to pick haha 😛 In that season Xander's role was turned down a WHOLE lot, and the only redeeming moments for me were when he did his speech to Dawn about ordinary peoples, and when he did that 'something' for Buffy. He didn't even get an episode centered on him(no I don't count 'First Date' as being about him). And they killed his love for no other reason than to kill someone for the finale.
Willow Rosenberg:
Favorite: Season two. This is funny because I understand her character like the others had to change and mature. But sadly, for me as her character evolved, she got less interesting and funny. I liked the shy, cute and randomly bitchy Willow! She was the best in season two(and part of season three), these were the good old days of the Scoobies 🙁 Her crush on Xander got me and then she meet Oz who was soooo right for her haha 😛 They were both able to be that KIND of awkward and smart and fun at the same time. Too bad they didn't stay and mature together(not that I dislike Tara, far from it!).
Least favorite: Season seven. Again, just like Xander, she was brought down a notch or two in the last season. Too many new characters took her screen time. I thought that after what happened at the end of season six, she'd be very important to the story, her being still touchy and prone to dark magic. Her role at the end was cool but still a bit too late to be that interesting.
Rupert Giles:
Favorite: Season four. Giles was my second fav character through the first five seasons. I liked him A LOT in each for multiple reasons, but... In season four, he was HILARIOUS! That's what I always loved about him, he could be stern and serious while still being sensible and funny. His relation with Spike was great and could have been used more later on! There was also couple of his funniest moments in that season. 'Something Blue', 'Hush', 'New Man' and 'Yoko Factor' haha 😀 God Giles was such a great character! Until...
Least favorite: Season seven. Again another Scooby having his worst season at the end(great...). Despite what some peoples said about this, I always thought Giles wasn't Giles in the last season. Gone(or almost) was his cute and funny side, gone was the touching relationship with him and Buffy. To me he almost turned into one of these council guys. It was all about the mission and the troubles ahead. There could be no fun and giggles as in the past. I also thought he'd have a much more important role in the end. Him being their father like image from the start, you'd think he'd be more than just a 'guy' amongst the group.
Anya 'Jenkins':
Favorite: Season five. Anya wasn't a character I liked much at the start. It took me a while to get warmed up to her. It's the innocent and snarky sides that got me in the end. In season five, she was at her best, 'Triangle' being to me her greatest episode. I loved how she brought that direct to the point and/or sarcastic side to the Scoobies(a bit like Cordy did in the first seasons). She showed pretty good human traits for someone who had been a demon for over a thousand years! She had a lot of great moments in that season, like her speech in 'The Body', her bitchering with Willow in 'Triangle' or when she lifted her arm in 'Family' and suddently asked 'What kind of demon??' hahahahaha! The smile she gave Xander then was so cute and hilarious 🙂
Least Favorite: Season six. She was alright at the start of the season but for me the worst part was after Xander left her at the altar. She was SO immature and annoying at that point. I got to say I despised things she did after that. Trying to get Xander's friends to hurt him. Wanting terrible things to happen to him instead of trying to talk about it with him. She sounded and looked like a little girl. the only thing I could kinda understand is when she had sex with Spike(this is out of topic, but I always wondered, how come they make us think Spike get drunk when he clearly state multiple times that vampires can't get drunk?). But yea, I know that immature side kinda was part of her character, it still irritated me to no end. It's not like she had not done terrible things before too, and Xander wasn't some jerk who did that to hurt her and left for good. He had his reasons and he tried to talk to her about it afterwards but to no avail.
William 'Spike':
Favorite: Season four. This was another hard choice for me. Contrary to some peoples, I liked Spike across all his 'phases'. I like the bad and dark Spike, I like the funny and pitiful Spike, I like the lovesick and heroic Spike. Now for me it was about choosing the one I preferred. And I picked the funny and kind of pitiful William 🙂 He was just GREAT in season four. He realized he couldn't hurt peoples anymore, was depressive about it(see scene with Willow LOL). Then he just hang around with them since he couldn't protect himself haha, so humiliating for the guy who thought he was the 'BIG BAD' (as he said himself when he came back to Sunnydale ::rolf:🙂
Least Favorite: Season six. Kinda like Buffy in that season, I thought some things he did were a bit out of character(given he was a soulless demon unlike Buff). Now this is pretty much it. I didn't dislike Spike in any seasons. He was a pretty good character, and since i'm gonna talk about AtS characters too, I got to mention he was very good in the last season over there! I could have picked it over this one.
Dawn Summers:
Favorite: Season five. YES season five! I know lots of peoples think she's annoying, immature and all, but to me this was the best season for her character. Dawn is not really a favorite character of mine. She's good as Buffy's sister but she never turned into much of a character that stood out for herself. Anyway, back to this season. I liked how they explored her view of what it was to be the sister of a Slayer. The whole journal thing at the start was great, and one of the FEW times I felt for her was when she finally found out she was the Key. She also had an interesting relationship with Spike in that season, kinda like a lil sister with her big brother. She bring in a different view of the main characters world. Yea she was annoying sometimes, but which teenagers aren't? (and I was too haha!) All in all, good job to the writers for getting her in with the main cast with a great storyline.
Least Favorite: Season six. Her purpose in the story was less clear for me at that point. She was still Buffy's sister and all, but she didn't bring much into the plot. I thought the episode 'Older and Far Away' had her act in a really weird and forced way(where did the whole no one care about me came out from??). I actually liked the whole being unable to get out of a house plot, but how Dawn was felt really forced and just to make this whole thing happen. There was no build up to it, at least from my point of view. In the end though, probably my favorite scene with her was at the end of this season when she showed Buffy she could fight and take care of herself if only Buffy would trust her.
Tara Maclay:
Favorite: Season four. Tara was a character that I just couldn't help but like. On the other hand she never really had an important storyline concerning her. The only episode centered on her was 'Family' and it was great, but i'm sure they could have done more with her role. Season four was my pick because it showed what she could bring to the whole group. She was working out really well with Willow and a bit like Xander with Buffy she was a great emotional support. Again, she never was THAT important to the plot but she was great for what she was mean't for. Her and Oz are the hardest for me to talk about since I just don't think they got as much development as the rest. Which is kinda sad.
Least favorite: None. Yes, none. She wasn't less than what she was in season four afterward. All I can say is that as said up there, I wish they would have expanded her character, probably in season six or seven(if they didn't kill her).
Daniel 'Oz' Osbourne:
Favorite: Season four. I really liked his character in that season. He finally got some character development with Veruca and I thought they'd expand on that, not just get rid of him. I know there was real life things linked to his departure, but since we're talking about the 'character'. He could have come back for good, weither Willow was with Tara or not, it's a bit like her(Tara) in fact, they never seemed to be developed much outside of the person they were in love with. I'd have liked more Oz centered stories on Buffy! He could have been a really cool character, being a werewolf and all 🙂
Least favorite: Season two. He came on late, and even though his Ozness was already present, we didn't see much of him that season 😛
AtS main characters:
Liam 'Angel' 'Angelus':
Favorite: Season three. I absolutely loved Angel in that season. We saw a new side to him(DADDY ANGEL) and it was both cute and hilarious! Also for me this is the most interesting season about the character. Getting the past(Holtz) to come back and haunt his newly found life(Connor) and happiness was cruel but SO fitting for Angel. He can never seems to get the 'reward' for his long redemption path. Every time, something come to destroy or steal it.
Least favorite: Season two. I know lots of peoples like the whole 'dark' Angel arc in that season, but I didn't. But again I can't say I hated Angel in any season on HIS show. So that's pretty much all I have to say there.
Cordelia Chase:
Favorite: Season one. This was another hard choice. Cordy is AWESOME in Angel, and the first three seasons are pretty even for her. I think I picked the first one because she had such good chemistry with either Angel and Doyle, and then Angel and Wesley! She's perfect for them 😀 She's so much more funny than in Buffy, and we also discover new sides to her, just like Angel! She matured, and was such a beloved character for me. I'm sad she wasn't in the last season.
Least Favorite: Season four. Alright, I know everyone on this planet(lol) HATE season four for what it did to Cordelia. But it wasn't the case for me. What I hated is the fact that in the end they killed her. I would have been fine with everything. But they COULD NOT kill her. Kill Wesley, kill Gunn, kill Fred, but Cordelia was way too important of a character to kill. She had to be with Angel till the end, she had to be part of W&H, she had to be part of the whole show. I'm really sorry Joss, but that was the biggest mistake you ever did on your shows, killing Cordy.
Wesley Wyndam-Pryce:
Favorite: Season four. Alright, like Spike I just loved Wesley every step of the way. He's such a cool character, so it was hard to pick the season I liked him the most in. But in the end, season four was his time to shine! First of all he was totally bad ass, he got really interesting to follow. He had so many great episodes. I liked his time with Lilah, he turned her into a character I actually liked(which was quite a feat trust me). He took pretty much charge of the group when Angel became Angelus. He and Faith together is a pretty awesome team in that season 🙂 We also saw the good old Wes in 'Spin the bottle'. Just great great great Wesley during that season!
Least favorite: Season five. Tough one. I hesitated between one and five. I picked five in the end because even though I said I liked him every step of the way, the way he became at the end really made me sad for him. He didn't seems to have any hope anymore, and he was an all around very grim character in this season(except for the short time with Fred). Again, i'm not bashing him. It's just isn't my fav part of him. Shame they killed him though... he had to be at Angel side for the final showdown, just like Cordy.
Charles Gunn:
Favorite: Season five. I thought for a while that Gunn's character lacked quite a bit of development, and personal storylines. So I really liked his story in the last season. He was not Fred's boyfriend anymore, he wasn't very close to either Angel or Wes. He found a use for himself outside of being 'muscle' for the group. I really felt for him when he realized how far that had got him though... He had took part in what killed Fred 🙁 Just made him re-think everything he had chose until then. And then he sacrificied himself in that hell dimension. I think these were the biggest moments for him. Overall he wasn't my fav character, but I was glad he wasn't that stereotypical black guy throwing lame jokes every minute(Eddie i'm thinking of you).
Least favorite: Season two. I wasn't a big fan of Gunn in s1 and s2. I actually thought the character was pretty blend and uninteresting. I was pondering at the start of the third season if I was every gonna like him(which I did from 'That Old Gang of Mine and on). Anyway, the season two was my less favorite for him. He didn't really have much personality I thought. He was part of the team and slowly grew closer to Wes and Cordy, but never really brought something special, except kicking some ass(which i'll admit, he was good for a human haha!).
Wini'fred' Burkle:
Favorite: Season four. I hesitated between three and four, but had to take four solely because of 'Magic Bullet'. That episode was reaaally great for me(excluding the fact it contained Jasmine). Poor Fred 🙁 She was all alone... Again, she's another character I didn't really dislike in any season. But I liked the whole love triangle Gunn-Fred-Wes in that that season. Even though she was really slow to get Wesley loved her, DUH! Also she was GREAT in 'Spin the bottle' hahaha 😛 Gogo Fred! I compare her a bit to Tara in BtVS except she was more active and actually kicked ass sometimes. They both were characters I just couldn't get myself to not like.
Least favorite: Season two. Ok, I know she was only in three episodes, but still. I would have never thought from that story that she would become important in the show. When I re-watch it, I look to see if Wes and Gunn even showed a sign of interest at first and nope! Kinda weird to me. Writers changed their plans later on haha? Oh well, that's pretty much it.
Krevlornswath "Lorne" of the Deathwok Clan:
Favorite: Season three. This sucked to pick a season... LORNEEEEEE!! Oh my god I love that guy, such a great character. Got to be totally honest, I never thought i'd like him after seeing the first episode he was in! But he's awesome. In season three he finally get with the group, he also play a big role around Angel, Connor and Cordy. He was like an uncle for Connor, a confident for Angel, which actually brought a side of Angel never seen in BtVS EH! He was the only demon of the group and even though that, he was actually the nice and non-violent guy. I just like pretty much everything he say, he was sarcastic as hell ala Cordy or Xander, he was also smart and threw couple brilliant lines all the time. He just made bad situations look better. Seeing how important he was to the whole team, I wonder why he wasn't introduced faster. My fav character on the show! Whoever came up with that character should be applauded.
Least favorite: Season five. And only because I had to pick one! He was still great but I thought he wasn't of much importance to the plot. He was detached for a while and didn't have as many brilliant moments(even though I love the time he's disguised with his sunglasses and spy on Illyria LOL). And then you got the moment I despise, Lorne kill Lindsey. Screw you Angel!!
I could have probably named a few additional characters from each show, but this was such a long post, I need to take a break now haha! Good luck to those taking the time to read it!