Villains - not that many who really do much for me - but Adam and Eve are the worst. Besides, that's fun to write! I think the mayor and Holtz were the most fleshed out.
As for cast - they all have flaws but some flaws annoy me more than others.
Xander bothers me most because he's a hypocrite, and because, until season 7, he refuses to grow up and take some responsibility. I just want to hit him upside the head so often. He's so selective in what he is willing to see and acknowledge. Like most of the characters, I kinda like him at first but he got on my nerves more and more. By the end of the show I just couldn't stand him! On the rewatches, I liked him even less.
Anya was a waste of a fantastic character idea, and I kinda think that was because they couldn't go into depth without making Xander see who she really was, and he was already a problem.
Riley annoys me for much the same reason - he blames other people for the choices he makes. There's a morality thing for me there, too. Buffy is the slayer, but she doesn't attack harmless demons, and she just kills the others. There's no imprisonment, torture, experiments - all that nazi crap. And the whole acting out and blaming Buffy just pisses me off.
Angelus is evil and enjoyable. Spike has a laundry list of things I would dislike in reality, and I really enjoy him too. Dru and Darla (Ats) are great. In fact, a lot of the characters I would really dislike in person, I love on the show. Lilah is excellent - she knows who she is and never makes excuses.
Angel can be a doofus, and then he can be scary focused and powerful. Cordy is fun - I really like how she evolved on Ats, both her caring and her snark, and then she left Pylea, and Groo came along.....and just bleh!
Gunn is good in theory, but I don't really feel it. Kate is a low rent Buffy wannabe.
Willow is realistic - I don't approve but I do understand her, and she does learn and grow. Besides, I really like her sense of humor, and her caring. Buffy - pretty much the same.
I love Giles - but season 7 pod!Giles, not so much. Early Giles - he is so wonderful that I have not the words to describe. I dreamed once that we were married, and it was a very nice dream. ASH is an amazing actor. So is Marsters - they never get predictable in their line readings - always fresh.
Wesley....Wesley is my complete package. I love Denisoff, and I love the entire Wesley arc. He gives me feels and more feels, and he can say more with a look than anyone who is not Wesley. In reality, I would be too intimidated to even talk to him.
In reality, I would love to have some tea with Tara, smoke some weed with Fred, and complain with Dawn.