For BtVS, there are SO many, but I'll name a few that come to mind that I don't think have been mentioned. Spike in Xander's Hawaiian shirt always gets me. Practically all of Doppelgangland is hilarious, though after seeing it 7 times, the laugh-out-loud moments are becoming less abundant. Willow's exchange with her mother in Gingerbread was just classic. And, of course, Spike's awesome goggles in Spiral. Now, some quotes:
Spike: (in a really bad American accent) Um, I'm just a friend of Xander's...
Willow: Oh, it's so cute! He balances a bunch of stuff, including the fish in the bowl...but, uh, don't try it for real when you're six, 'cause then you're not allowed to have a fish for five years.
^ totally sounds like something I'd try!
For Angel, there are a lot (mostly from Spike, Harmony, and Lorne), but I just watched Life of the Party, so I'm gonna bring up one of my all-time favorite moments...when the whole dance floor is completely empty except for Harmony, who's dancing in the middle. It's especially funny to me, I suppose, because it reminds me of someone I know. Also from Life of the Party, this line from Spike:
Spike: You pissed in the big man's chair? That's fantastic!
Gunn: Spike, could you turn off the warm fuzzy?
Spike: What, the Lorne thing? Wore off. I just think that's bloody marvelous.
And how could I forget the famous...
Illyria: I'd like to keep Spike as my pet.