Xander: "Well, we could ground up our enemies into talcum powder with a sledge hammer, but gosh, we did that last night."
Buffy: "You have 'but' face."
All those hilariously-placed cut scenes...like in Dead Man's Party when Xander shouts, "You can't just bury stuff, Buffy. It'll come right back up to get you." And then it cuts to a zombie rising from the ground. Or in School Hard when Spike says, "So, this Slayer. Is she tough?" and it cuts to Buffy combing her hair saying "Ow." Those are always funny.
Oh, and in Dead Man's Party when Willow says, "Why not? Talking isn't doing anything, we might as well try some violence." Then the zombies burst into the house and break that random guy's neck, and Willow yells, "I was joking!"
Harmony's gang always cracks me up..
Andrew zooming in on Xander's woodwork instead of Willow and Kennedy kissing on the couch.
Giles: "It's the end of the world."
Everybody: "AGAIN?"
When Anya comforts Giles in OMWF by robotically patting his shoulder. Or when she pats Willow's shoulder before Willow goes to delve into Buffy's mind and says, "Good luck!" mechanically. Anya just cracks me up.
The Buffybot. She's hysterical, SMG acts splendidly with her.
Giles: "See if you can wrest information from that blast machine. *pause* That was very British, wasn't it?"
(Can't remember exact quote)
Giles: "Oh, dear."
(Everyone goes on talking, ignoring him.)
Giles: "I said oh dear!"
Buffy: "You always say that."
Giles: "It's always important!"
I could keep going and going...BtVS always has me laughing, I find it so funny.