From the Season 02 episode 01: When She Was Bad
I laughed out pretty hard when the gang was discussing Buffy's latest "change" in personality during the summer break, and then ending the conversation with:
Willow:
"That's what it was! 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 bitcuh?" Also, as a fan of Miami Vice, I loved the little dig at the show and its fashion sense, (not sure which episode) but the moment some of the vampires arises from the "down under", some of them looks like they are dressed up like Don Johnson and is an easy prey for the vampire hunter when arriving at the Bronze night club, and Buffy looks at one of them and says:
"Okay, first of all, what's with the outfit? Live in the now, okay? You look like DeBarge!" Yes, I think that the 'bitca'/'bitcuh' exchange is certainly a classic.
Regarding your second piece of dialogue from the show, I never actually knew who 'DeBarge' was when Buffy said that, but given the way she referenced him, I could infer her meaning.
It’s far too difficult to pick what could be deemed the ‘funniest line’ from one of the most quotable shows ever, as what’s considered ‘funny’ is all subjective really. These are probably by no means regarded as the ‘funniest’ lines from the show, but they’re the ones I always remember and quote the most in my head at random times:
From Season 2’s ‘Halloween’ Oz: "Hey, Cordelia. Geez, you're like a great big cat."
Cordelia: "It's my costume. Are you guys playing tonight?"
Oz: "Yeah, at the Shelter Club."
Cordelia: “Is Mr. I'm-the-lead-singer-I'm-so-great-I-don't-have-to-show-up-for-my-date-or-even-call gonna be there?”
Oz: “Yeah, y'know, he's just going by 'Devon' now.”
From Season 3’s ‘Dopplegangland’ Xander: “Willow, did you remember to tape Biography last Friday?”
Willow: “Uh-huh.”
Buffy: “See, I told you. Old Reliable.”
Willow: “Oh, thanks.”
Buffy: “What?”
Willow: “'Old Reliable'? Yeah, great. There's a sexy nickname.”
Buffy: “Well, I-I didn't mean it as...”
Willow: “No, it's fine. I'm 'Old Reliable'.”
Xander: “She just means, you know, the geyser. You're like a geyser of fun that goes off at regular intervals.”
Willow: “That's Old Faithful.”
Xander: “Isn't that the dog that, that the guy had to shoot...”
Willow: “That's Old Yeller.”
Buffy: “Xander, I beg you not to help me.”
From ‘Graduation Day, Part 2’ Buffy: [to Angel] “You and Xander are going to have to work together now. Can you guys handle that?"
Xander: "But I'm still key-guy, right?"
Buffy: "Right."
Xander: "Then Angel, in his non key-guy capacity, can work with me."
Angel: "What fun."
Xander: "Hey! Key-guy is still talking..."
From Season 4’s ‘The Freshman’ Xander: “Buffy, this is all about fear. It's understandable, but you can't let it control you. 'Fear leads to anger. Anger leads to hate. Hate leads to anger.' No wait, hold on. 'Fear leads to hate. Hate leads to the dark side.' Hold on, no, umm, 'First you get the women, then you get the money, then you...' okay, can we forget that?”
Buffy: “Thanks for the Dadaist pep talk, I feel much more abstract now.”
And... Xander: “You up for a little reconnaissance?”
Buffy: “You mean where we all sculpt and paint and stuff?”
Xander: “No, that was the renaissance.”
Buffy: “Oh. I've had a really long week.”
From ‘Doomed’ Buffy: [to Giles] “I told you! I-I said end of the world and you’re like ‘poo-poo southern California, poo-poo!'”
From Season 6’s ‘Bargaining, Part 1’ Giles: “You might have let me in on your plan while he throttled me.”
Spike: “Oh, poor Watcher. Did your life pass before your eyes? Cuppa tea, cuppa tea, almost got shagged, cuppa tea?”
From Season 7’s ‘Chosen’ Xander: [after Buffy says she cut Caleb in half] “Hey, party in my eye socket and everyone's invited.” [pause] “Sometimes I shouldn't say words.”