Spike/Dawn: Also one of my favourites. You're right with the "uncle" thing, it was really neat to watch. He was always looking out for her.
Willow/Xander: Always loved the kind of friendship that these two had (especially in seasons 5-7). Buffy was close to them but not in the same way, and I loved those scenes where their friendship really came out.
Joyce/Spike: As has already been said, they were just funny. Passions and girl trouble woes... *sighs* hilarious! 😀
Giles/Buffy: The father/daughter relationship with these two was always one of my faves. Especially in "Checkpoint" and when Giles returns in season 6, you really get to see just how close they had become. I'm blissfully ignoring the part where they have their tiff's in season 7, but even that happens with parents/children sometimes. With the action of the show, it was easy to forget how close those two were, Giles really was more of a father to Buffy (I think) than her Dad was ever shown to be.