Buffy’s most significant experience?

Killing Angel. That's the one she refers back to - rarely explicitly, but still - in ever difficult decision afterwards.

Willow’s most romantic moment?

None of the above. The Tara one is just too ominous. I'd go with the ending of "New Moon Rising" (all of it).

Xander’s favourite holiday event?

I said snow, then I figured that it meant the poor guy had to go back inside, so I'm going to change my vote to s6 Halloween.

Angel’s most satisfying piece of revenge?

Spike. Because that's the one he'll never ever in a million years feel bad about. :-)

Spike’s favourite fake relationship with Buffy?

Faith. C'mon. You know that girl just filled his poor head with ideas it hadn't even thought of before.

Giles’ favourite job?

Librarian in a library which only four kids seemed to know existed.

Buffy’s best boyfriend?