There is only ONE winner for this.
Donkey Kong Country 2 - Forest Interlude
Forget favorite video game music, this is my favorite piece of music as a whole, in any medium whatsoever. It is wonderfully composed, engrossingly atmospheric and just beautiful to listen to - and considering the SNES sound limitations, this is nothing short of perfection. The whole game is full of fantastic soundtracks.
Hot Head Bop
I was actually on a track to post every single soundtrack in the game so I'll stop here lol.
Donkey Kong Country
Rhythm Tengoku (The sound quality here is actually REALLY impressive for a GBA game)
Rhythm Heaven Fever
Elite Beat Agents (I know they're covers but they're so good.)
Wario Land 4
Super Mario Land
Punch Out!! (Wii)