This question varies from Season to Season, but over all: Anya. Absolutely, no question. I mean come on, I love this little exchange:
Xander: Just once, I'd like to meet a cult of bunny-worshipers.
Anya: Great. Thank you very much for those nightmares.
Andrew was like, my light during Season 6, because he was just so freakin' cute and amusing. Then he got even better during Season 7. My favorite scenes from Season 7, then is the combo of my 2 favorite comedic characters. Any scenes with Anya and Andrew make me laugh my butt off.
example: The scene in the hospital. The wheelchair fight was classic. And this line, from Chosen:
Anya: No one cares, you little monkey.
Of course Spike is always fun, especially during Seasons 4 and Five.
I also found Tara's sense of humor incredibly endearing. She's like me, she just makes random observations that are worded just a little differently than you might expect. And they're often incredibly insightful. I mean, Miss Psycho Pep Squad? That's great!