Buffy and Angel all the way. Hey, they spent what, four seasons apart and never forgot about each other. Buffy let angel suck her blood to save his life. Angel was willing to fight through an apocalypse with Buffy--not to mention giving up his humanity for her. That's pretty special.
I never really understood Buffy and Spike. It's not that I thought he was some kind of replacement, because he was obviously a lot more. They just never clicked for me.