If it weren't for Xander, I'd say Anya, but Xander's there... I say Drusilla. They were awesome together and had so much chemistry. Spike and Buffy have good chemistry, but I am not happy the way their relationship turned out.
S/W is an interesting theory and that was good evidence that you provided. I can actually see them together. That episode was so funny.
Willow: Let's wait a half an hour and try this again. Or...not."
Then she hits him with a lamp! 😀