If I'm only counting full playthroughs, then it's not that many games, because of my habit of often switching off as I reached the final boss after I'd beaten them once or twice.
I chose Sonic 3D Blast, which I played on the Genesis.
It's not the best of Sonic games; the isometric gameplay is a little weird, and I was constantly having to backtrack to look for a "flicky" bird that I had left behind, but I somehow found it worth constantly revisiting. It's a similar format to Sonic Labyrinth, only more bearable.
I enjoyed the BGM, too.