I have to step in an defend the Megadrive sound here. While all machines have good and bad sound (and bad Megadrive sound was really dire), the machine could produce some awesome music.
Super Shinobi, Ghouls 'n' Ghosts, Streets of Rage for example.
Technosoft were masters of that sound chip and their games always sounded amazing (aside from Devil Crash). My particular favourites are Herzog Zwei and the game complete tune from Elemental Master.
I'm not sure why people like the sound in Sonic so much though...I always thought it was flat and unimaginative.
One special mention should go to Sunsoft's Batman on the megadrive. I just love that soundtrack.