Second, I think, to Sonic '06, but it's pretty damn close
Genesis doesn't count due to most of its music being remixes
Yeah Sonic 06 had contained some of my favourite sonic music too.
Little confused on the Genises music being mostly remixes though. I'm aware in Sonic 3 MJ based segments of the soundtrack on his pre-existing music and one other artist based the entire zone music on one of their already existing songs like this one:
Even then a few zones had entirely new pieces of music written for them?
I'm sure however Sonic 1 and 2 had soundtracks created from scratch. Obviously there's the demo music which many composers created before making the 16 bit version as it's easier to create a tune when not confined to Mega drive instruments.