My first thought was Tara/Willow, they had their problems but they always worked through those. I think s4-5 they were both really good for each other and brought a lot of growth for each other. S6 was a bit of an abusive mess with the magic, but in the end, an addiction can only be changed by the user. Sure, supports helps, but someone only changes when they want to change themselves, that is a journey only Willow could take. And in the end, it's only by bettering herself that Willow could change her behavior and make it work with Tara. I don't think having Oz there would have changed anything. In s6, magic was portrayed as an addiction, and no amount of love or support could change the fact that Willow had been on this path as early as s2-3. That is without saying that Willow was already dealing with some sign of magic abuse in s4, some of which was in relation to Oz (not that it was Oz's fault at all, but Willow would have abused magic at some point or another, as soon as issues arose in her relationship. That is something she does both with Tara and Oz, which to me shows that the partner really didn't matter, it was always about the magic). That being said, as far as highschool years, Woz was pretty great.
I would also say Willow and Kennedy was a great ship. Say whatever you want, but Willow was good for Kennedy, and more importantly, Kennedy was good for Willow. Most people hate Kennedy, but no one can deny that she was a good girlfriend (I dare anyone to try and make a good argument against this).
From what I've seen from s7 and s8, I liked Xander and Dawn and pretty much think they make a great couple, although I haven't read the comics far enough to see where this goes.
The Fanged Four is pretty much vampire goal as far as relationships goes.
Gunn and Fred were good at the beginning. As were Angel and Cordy.
Giles and Jenny is another good one.
For the most part, I think Buffy doesn't make good choices in relationship, her best relationship was with Riley and even that was a mess. As for Xander, I didn't really see any major character growth related to his relationship, and by s6, he basically hasn't matured enough to even be in a good relationship (I don't really blame him considering he's only 21).