I love the sound of the Sega Mega Drive. My favourite soundtracks, therefore, are from Sega games.
My absolute favourite, for the sake of nostalgia and not necessarily the best, is Sonic the Hedgehog 2's soundtrack.
Pretty much the soundtrack of my childhood. I've heard it hundreds of times and it's still some of the best VGM there is, imo.
Second, and which I consider to be the single best VG soundtrack ever produced, is the Ecco the Dolphin and Ecco: The Tides of Time soundtrack. inspired by the music of Pink Floyd, more specifically the album Wish You Were Here, the soundtrack to Ecco is... mind blowing. I quite enjoy listening to it, on it's own, in order, with no gameplay. This is not typical for me, but it is so good that it's practically a musical album anyway. It's soundtrack fits perfect with the gameplay and plot, which is straight out of a psychedelic trip. Completely crazily brilliant soundtrack.
Third is the soundtrack to Kid Chameleon. Not a whole lot to say about this one, other than that it is very good