Specifically (partially memory, partially the shooting script site, so I apologize for any slight inaccuracies) :
XANDER: Where is he? Where’s the creep who turned me into his spider-eating man bitch?! I’ve got a flaming enema with his name on it-
BUFFY: He’s gone.
XANDER: Damn it!.. You know what? I’m sick of this crap. I’m sick of being the guy who eats the insects and gets the funny syphilis! As of this moment, it’s over. I’m finished being everybody’s butt monkey!
Some personal favorites (God, I love ‘Something Blue,’ and all of Joss Whedon’s dialogue.) :
(Something Blue)
BUFFY: Honey, we have to talk about invitations. Do you want to be “William the Bloody,” or like, just “Spike?” 'cause either way, it’s gonna look majorly weird.
SPIKE: Whereas the name “Buffy” gives it that touch of classic elegance.
BUFFY: What’s wrong with Buffy?
GILES: Such a good question.
GILES: It’s alright. I have more scotch.
(Innocence)
OZ: So, do you guys steal weapons from the army a lot?
WILLOW: Well, we don’t have cable, so we have to make our own fun.
(Earshot)
WILLOW: So, you’re feeling better about Angel.
BUFFY: Yeah, I mean we talked, and he ripped the heart out of a demon and fed it to me, and later we talked some more.
WILLOW: See, that’s how it should work.
(The Freshman)
SIDEKICK VAMP: Are we gonna fight, or is there just gonna be a monster sarcasm rally?
XANDER: Do we hug?
OZ: I think we’re too manly.
WILLOW: What’s a prank?
XANDER: Prank? Oh! The room! Some friends of Buffy’s played a funny joke, they took her stuff, and now she wants us to help get it back from her friends who sleep all day and have no tans.
WILLOW: Oh, those friends.
OZ: They’re funny guys.
Not dialogue (obviously,) but the whole overhead projector scene from ‘Hush’ deserves to be mentioned on general principles.
Lots of funny lines from the show Angel too. Is that another thread?