I can't pick one. Instead, I shall list a whole bunch of my favorite quotes.
Buffy: Bright, sunny, beautiful. However can we escape this torment?
~Teacher's Pet
Buffy: If the apocalypse comes, beep me.
~Never Kill a Boy on the First Date
Willow: I mean, why else would she be acting like such a b-i-t-c-h?
Giles: Willow, I think we're all a little too old to be spelling things out.
Xander: A bitca?
~When She Was Bad
Spike: I was actually at Woodstock. That was a weird gig. I fed off a flower person, and then I spent the next six hours watching my hand move
~School Hard
Buffy: Does it ever get easy?
Giles: You mean life?
Buffy: Yeah. Does it get easy?
Giles: What do you want me to say?
Buffy:Lie to me.
Giles: Yes, it's terribly simple. The good guys are always stalwart and true, the bad guys are easily distinguished by their pointy horns or black hats, and, uh, we always defeat them and save the day. No one ever dies, and everybody lives happily ever after.
Buffy: Liar
~Lie To Me
Whistler: Bottom line is, even if you see them coming, you're not ready for the big moments. No one asks for their life to change, not really. But it does. So, what are we, helpless? Puppets? Nah. The big moments are gonna come, you can't help that. It's what you do afterwards that count. That's when you find out who you are.
~Becoming
Buffy: I hate you
Spike: And I'm all you've got
~Becoming
Angelus: No weapons, no friends, no hope. Take all that away, and what's left?
Buffy: Me
~Becoming
Spike: You're not friends. You'll never be friends. You'll be in love 'til it kills you both. You'll fight, and you'll shag, and you'll hate each other 'til it makes you quiver, but you'll never be friends. Love isn't brains, children, it's blood -- blood screaming inside you to work its will. I may be love's bitch, but at least I'm man enough to admit it.
~Lover's Walk
Willow: Bored now
~The Wish, Doppelgangland, Villain
Willow: ...And I think I'm kinda gay
~Dopplelgangland, Tabula Rasa
Buffy: You had sex with Giles? You had sex with Giles?! On the hood of a police car? Twice?
~Earshot
Willow: What are you doing?
Oz: Panicking.
~Graduation Day
Xander: Who's a little fear demon? Come on! Who's a little fear demon?
Giles: Don't taunt the fear demon
Xander: Why? Can he hurt me?
Giles: No, it's just tacky
~Fear, Itself
Anya: To commemorate a past event, you kill and eat an animal. It's a ritual sacrifice... with pie.
~Pangs
Spike: I just can't take all this namby-pamby boo-hooing about the bloody Indians.
Willow: Uh, the preferred term is-
Spike: You won. All right? You came in and you killed them and you took their land. That's what conquering nations do. That's what Caesar did, and he's not going around saying, "I came, I conquered, I felt really bad about it." The history of the world is not people making friends. You had better weapons, and you massacred them. End of story.
~Pangs
Giles: It's the end of the world.
Buffy, Willow, Xander: Again?
~Doomed
Faith as Buffy: You can't do that. It's wrong.
~Who Are You?
Spike: My heart expands. Tis grown a bulge in it. Inspired by your beauty effulgent
~Fool For Love
Anya: But I don't understand! I don't understand how this all happens. How we go through this. I knew her, and then she's— there's just a body, and I don't understand why she just can't get back in it and not be dead... anymore! It's stupid! It's mortal and stupid! And... and Xander's crying and not talking, and... and I was having fruit punch, and I thought, well, Joyce will never have any more fruit punch, ever, and she'll never have eggs, or yawn, or brush her hair, not ever, and no one will explain to me why!
~The Body
Buffy: The hardest thing in this world is to live in it
~The Gift
Spike: Every night I save you
~After Life
Andrew: Timothy Dalton should win an Oscar and beat Sean Connery over the head with it!
~Life Serial
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There are probably some I missed, but that's ok.