I've never considered the Genesis/Mega Drive sound chip to be particularly good when compared to the SNES, but some games either make them sound good or are just well-suited to the unique Genesis sounds.

Ghouls 'n Ghosts' eerie levels combine well with the music, which has a kind of disturbing feel to it. The Crystal Forest might be the best example of this; filled with weird sounds and creepy effects, it just gels well the raw, almost ugly tones of the sound chip.

A nice contrast in the OST are the almost happy melodies, like the whimsical opening of The Village of Decay and Destruction.