Musical designers and remixers (since some tunes are remixes, like Jade Valley Zone which is a remix of Sunset Park Zone) did a great job on the musics. The tracks are very balanced and magical to listen to. Some cool themes like ERZ's stayed and still stay in my head, and most of them (almost all, actually - except SRB1 remade in most of the cases) greatly fit to their levels. My personal favourites musics are especially the ones from the 1-Player stages, probably because the music attracted me a lot when I started on SRB2, and that I discovered how to fix this online mode far later.
Too bad that now, I'm almost "used to the tracks", so I can't feel them as much as when I listened to them for the firsts times...
Um anyway, to answer, my favourite tune is actually Greenflower Zone Act 1, and the Act 2 in second place. These songs kind of reflect a friendly first stage, but also the peaceful tranquility, and why not the hope of Sonic's adventure ahead of him.