For some reason, I've always loved the line, "That's where they make Gileses" in I-can't-remember what season.
And, although I detest Riley, I love Buffy's, "My hat has a cow" in As You Were.
Also, in First Date, this whole exchange always amuses me:
Buffy: On the Hellmouth. All day, every day. That's gotta be like being showered with evil. Only from underneath.
Willow: Not really a shower.
Buffy: A bidet. Like a bidet of evil.
And to wrap things up... "I've got to warn Buffy. There's every chance she might be next. And Xander, help Willow, and try not to bleed on my couch I just had it steam cleaned."
And OH MY GOD! I guarantee no one else will find this funny, but my friend and I have this ongoing inside joke that we use far too frequently, but basically, there's one of Riley's lines in season 4, I can't remember which episode, but Buffy's clearly going through some internal emotional termoil, and Riley just shoves his giant, donut holding hand in her face and asks, "Jelly?". Whenever one of us is texting the other about going to get food, we just write 'jelly' with a question mark.
Not to mention every poem Spike's ever written... Which, I, ahem, may or may not have memorized to whip out at a moment's notice...