Rather than quote the entire movie (which would be the only way to get down every excellent line) I'll just transcribe some of my very favorite exchanges. Here we go:

[After Buffy has spent a night Slaying...]
BUFFY: It was a trap, get it? I led him in.
MERRICK: It was a blind alley, get it? If there had been any more of them you would be dead. You must never forget the cardinal rule Buffy: One Vampire is a lot easier to kill than ten.
BUFFY: Does the word 'duh' mean anything to you?

[After Amilyn comes back from the fight where he lost his arm in Pike's truck]
AMILYN: I had him in my grasp!
LOTHOS: Cheer up, you may still.

[While Buffy's fighting the one-armed Amilyn]
AMILYN: We're immortal Buffy. We can do anything!
BUFFY: (kicks him up against a wall) Oh yeah? Clap.

[Buffy talking with Merrick about Slaying]
BUFFY: Merrick, I'm not gonna croak that easily. I've got something those other girls didn't have.
MERRICK: And what might that be, pray?
BUFFY: (shrugs) My keen fashion sense?
MERRICK: Oh, Vampires of the world beware.
BUFFY: Merrick, you made a joke! Are you okay, do you want to lie down? I know it hurts the first time...

[In an interview after the slaughter at the Senior Dance]
GARY MURRAY: I think the students learned a valuable lesson. Except for the dead ones.. I mean, they learned it, they just didn't really have time to implement it... are we live?

[Pike and Benny, somewhat inebriated, discuss Buffy and friends]
PIKE: I mean, you don't even like her, but you'd sleep with her. What is that?
BENNY: I got news for ya, pal: another shot of this and I'd have sex with you.
PIKE: Yeah, but you'd never call me...

[Merrick, trying to convince Buffy she is the Slayer, throws a knife at her, which she catches]
BUFFY: You threw a knife at my head!
MERRICK: Yes, I had to show you.
BUFFY: But... you threw a knife at my head.

[After Pike jumps into a fight and helps Buffy]
BUFFY: What are you doing here?
PIKE: What am I doing here? I'm saving your butt. (Buffy reacts shocked) Well... there was sort of an exchange of butts. At least I didn't faint.