I have a three-way response to this answer. Originally, I was thinking it was a 3-way tie between Xander/Willow, Xander/Buffy and Willow/Buffy, but in thinking about all the ways I love each friendship individually, I love the friendships they share in their little trio.
Xander is the civilian of the group, with no special powers, but he’ll do anything for his friends. Before Buffy came along, it was Xander and Willow, and they were so close. That of course was in the days before Xander realized his usefulness.
Perhaps it’s because Xander is one of my favorite characters anyway, but I love the interactions between the original Scooby Gang. They just go through so much. They saved the world. I don’t know what type of friendship would be better than that.