Spike/Angel would be total Sexy Overkill. That's a whoooole lotta man-pretty for one sex scene. Plus I imagine the aftermath for them would include a tenfold increase in bickering and nitpicking. Which would be fun.
I can't say Spike/Xander appeals to me . . . though seeing the aftermath sure does! Xander overcompensating like it's going out of style while Spike does everything he can to remind the whelp! ("You know you did it, and you can't bloody take it back, bricklayer. So there!")
Original S3 Buffy Wesley/Angelus--not Angel, ANGELUS. Pure Evil? Meet my pal goody-two-shoes. Shag him rotten if you wouldn't mind, he needs it. Seriously.
Mr.Trick/Faith. I think they would have seriously had fun trying to out-badass each other.
Veruca/Buffy. Veruca was that singer chic who seduced Oz. Hey, Buffy slept w/a girl in the comics anyway. If she dug athletic sex with a girl, imagine how much fun she'd have with a werewolf girl. Plus, semi-evil? Totally her type.
Ripper/Dru. She may not be OUR Giles's type, but I'll bet anything that Ripper would have had a grand old time with her back in the day. Yes, I am sick.