"I'm not sure if I'm driving this car or wearing it!"
Spike while driving Giles the Fyarl demon around.

"Stupid limey peice of crap" Puppet Angel to Spike

"the slayer is going to kick your skanky lopsided ass..."
Intervention

The look on Spike's face when he sees Buffy alive on the stairs in Afterlife.

"Cuppa tea, cuppa tea, almost got shagged...."

"Ooh look at my mask, isn't it pretty" Giles about Joyce's zombie raising mask in Dead Man's party.

Angel trying to find his Irish accent in Spin the Bottle.

"Me? Cheating? He's got X-ray vision!"
"I wasn't using it" Kitten Poker

And almost every single thing that others have listed. This has been an exercise in joy. Thanks.