The main one for me was Buffy and Xander. They were great as best friends, but I always thought that Xander was perfect for Buffy. Additionally, Xander always had more respect for Buff than his own girlfriends, who he often treated badly. My only fear was that he might end up acting the same way with Buffy, even though that would have surprised me considering the utmost esteem he had for her.
Of course there was also Buffy and Faith. Those two had a really good chemistry in S3, and in some scenes you could cut the sexual tension with a knife. Why didn't Faith kiss Buffy on the lips in 'Enemies'?! Arghhh, way to disappoint all the fans. 😉