I'm two discs into Buffy season 7 and Angel season 4. So at this point:
1. Buffy. She's what made me watch the show in the first place, and I still feel for her two discs into my dvd set of season 7. I can't believe I'm almost done with this show

2. Wesley. He's changed sooooo much from the prissy little posing Watcher he started as.
3. Giles. He's the man. Seriously, he's soooo cool. Buffy's figurative dad, well, actually, the Scoobies' dad, and hilariously British yet still a total badass. He just has so many good moments in the series.
4. Angel. So intricately tied with Buffy, with the mythos of the show. He's a great character himself. Like Buffy, it's harder to pinpoint what I like so much, it's just that he's the focus of his show, I guess. It helps.
5. Xander. Funny, likeable, fish-out-of-water, a regular guy surrounded by superpowers. He's out of his depth, yet essential to his superpowered friends because he grounds them.
6. Spike. Badass, funny. Got some character depth with the whole Buffy relationship.
7. Faith. Eliza just eats up the screen whenever she's in an ep. What a great, troubled character.
8. Cordelia. Evolved as much as Wesley.
9. The Mayor. Great baddy. '50s values and yet horribly evil. He was also a pretty good mayor (I remember him checking the shape of the pipes in the sewer system while down there en route to a nefarious deed).
10. Lorne. He's fab
I wish Willow was on the list but ah well. Too many good characters from the Buffyverse for a mere top 10.