The Master - Pretty lame, but fills his role as a villain well, with a clear motive. Had a few good lines ("What do you think, 4.2?") - 6/10
Drusilla - Awesome villain, her craziness was very entertaining - 9/10
Spike - The epitome of awesome in S2. He was the most entertaining character ever - 10/10
Angelus - Very creepy, very dangerous. Probably the best "Big Bad" there was. His connection to Buffy made him really interesting - 10/10
Faith - She is one of a my favourite characters, but if we only count her time being evil, then she wasn't as entertaining as other times - 9/10
The Mayor - Very entertaining, very funny, and his relationship with Faith was very sweet, he was a really well-rounded character. However it bugged me that he was never given a real motive, he just wanted to be a big snake for some reason or another - 8.5/10
Adam - Quite lame. His philosophical rants were quite interesting, but he was never really engaging - 6/10
Glory - A good villain with a clear motive, I enjoyed her but did think that her storyline dragged on a little too long - 7/10
The Trio - Truly lameass villains, They were supposed to be funny, but the supposed hilarity was that "they're nerds and they like Star Trek and live in basements". Truly hilarious. We were supposed to be entertained by these half-assed cliches? - 3/10
Warren (counted seperately from the Trio) - Actually a very good villain, completely convincing and completely hateable - 8/10
Dark Willow - Had potential, but really, the whole "magic addiction" failure of a plotline ruined it before Dark Willow even turned up. Her dialogue was turgid and cheesy, and she spent an awful lot of time doing nothing, her story seemed dragged out for two episodes, and then rushed at the end. - 5/10
I've only seen half of S7 so far, but:
The First - Interested me greatly at first, a creature that could appear as dead people and manipluate the living was a great idea in theory. However I'm half-way through the season and it still hasn't really done much. For an all-powerful manifestation of evil, it spends an awful lot of time talking and not nearly enough time doing.