Xander, Anya, and Willow are my top three. Xander was simply a goofy character. Anya had that whole "I'm an ex demon who doesnt understand being human" thing going. Willow, although not consistently funny she has a lot of great funny quotes.
"Ms.Calender:Morning, England.
Giles:Oh, hello, Ms., uh, Jenny.
Willow:Feel the passion.
Jenny:Willow...
Willow:Coughing, not speaking.
Willow:See, you made him do that thing where he's gone.
Xander: Why do I need to learn this?
Willow: 'Cause otherwise you'll flunk math.
Xander: Explain the part where that's bad.
Willow: You remember. You fail math. You flunk out of school. You end up being the guy at the pizza place that sweeps the floor and says, 'Hey kids, where's the cool parties this weekend?' We've been through this.
Buffy: Willow, you're a gay woman... and *he* isn't.
Willow: This isn't about his physical presence, it's about his heart.
Anya: His physical presence has a penis.
Willow: I can work around it.
Xander: Once you get back the soul, doesn't that mean you start, like, picking up your own wet towels off the floor?
Willow: No, but maybe you start to feel really bad about leaving them there.
Anya: Well, you're gonna have to do better than that. I'd kill for him.
Willow: You'd kill for a chocolate bar. "