Probably Final Fantasy Legend 2 or Zelda: Link's Awakening. I still have midi-format versions of those sitting around.
I wouldn't specifically recommend that someone go back and dig them up, though, if they haven't played them and aren't just wanting the nostalgia value.
For the Game Boy, I think that the technical limitations on the audio hardware really keep it from being competitive with larger systems, though. I mean, it's impressive what they managed to do with the hardware in that era, but the SNES was a lot more capable.
I really love the GameBoy and SNES sound with the caveat that its with equalizer that reduces the shit out of the treble and higher mid. Balanced out, its pure ear and brain candy 💥