Anya/Giles, for sure. In just one episode they managed to have more chemistry than most other couples (including Xander/Anya) in the rest of the show. I also think it could've been an interesting pairing, due to their physical age difference and then their mental age difference, as well. It always struck me as odd that an 1120-year-old former demon would fall for a teenage boy (18, but still). Especially as Xander is on the more immature side when they fall in love in S4.
The Anya/Spike one never made sense to me, the same way that Faith/Spike doesn't make sense to me; they are just far too alike. Both are demons that can no longer be what they were but don't really fit properly with humans, at the same time. Both understand the world and the people around them more than most and aren't afraid to point out the things they see to those people around them. I think it would be like dating yourself, with those two.