Buzz and Woody have the best friendship in Pixar in my opinion, the others barely come close to it. They may disagree and everything but they work things out realistically. They have a dynamic which means they rely on each other but they also call each other out when out of line. They’re not afraid to do that for each other and say what the other might not want to hear (Buzz is very good for Woody in this). They… are like the head and the second in command in a tribe almost and it just really WORKS.
They started off as rivals (or at least from Woody’s end) and they’ve had some rough times, times even when they left each other. They forgive though and work things out (Mike and Sulley issues sometimes seemed skated over and not natural really to me. There’s still work to do there for various reasons I won’t go into right now, so as not to harsh others parade.) Once again rather like Bob and Helen for me as a favourite ROMANTIC couple, I think there’s is the most realisitic and one we can relate to in terms of positive and healthy friendships. Even if they are toys. You can expect in the future disagreements and such but you kind of are reassurred that even when that happens they’ll be there for each other in the end.
Toy Story 2 is what really brought this home for me though, seeing how determined Buzz was to get to him and his slightly harsh but necessary words to Woody, much as I loved the first. And while there was a little less in TS3 (due to more characters probably among other things), I did like what I saw of it.