Season 1: I get why it's nobody's favourite, but people get down on it so much and I don't think it was that bad. It was very demon of the week, but it was funny and interesting and introduced the characters and their motives very well. It lacked some production value but that's budget not incompetance.
Season 2: Used to be my favourite season when I was a Bangel, and I still like it a lot, but now I appreciate Angelus a lot more and the triangle between the 3 vampires. I didn't like how much Buffy blamed herself for Angel going evil and I didn't like anyone else implying it was her fault. She slept with her boyfriend, it was going to happen sooner or later she couldn't have known what was going to happen.
Season 3: Love Faith and love the season. It's not very strong with stand alones but the arc is great and the relationships between all the characters are now very dense and interesting. Also what's more badass than all the students fighting at graduation?
Season 4: Probably the best season for stand alones, probably becuase the arc lacked a little something something. I don't really start to hate Riley until the end of the season so it's all okay there. Not much else to say except hilarious/wonderful episodes.
Season 5: Glory herself left a little something to be desired for me, but I really like the arc anyway. I also found it interesting (clearly I'm now a Spuffy) that Spike fell in love with Buffy. Especially because it just seemed to make sense. Joss always says he realized that Spike was in love with Buffy, it wasn't just contrived he just realized it, and I totally get it. I also love the Dawn storyline. Like many others I hated Dawn in the beginning but she has grown on me and I relate her a lot more now, and I relate to Buffy a lot and I get why she loves her so much.
Season 6: So from what I can tell is that people love it or hate it. I think it's good. I'm not a Spuffy person that's like "Yay they get it on!" I think they're relationship was problematic at best and horrible at normal, but still important. I think it was important to give the scoobies real problems that people face in life. And those nerds were effin' hilarious.
Season 7: This is what I picked as my favourite. Mainly because I can't choose and this is the best season for Spuffy. I think they left a lot of storylines unfinished and that bugged me but all in all it came full circle and was a great season.