Dang it's hard to choose ONE. I'm going tot do them by game...
Zelda I: Main theme (duuh) Dungeon theme is close second
Zelda II: Dungeon Theme (SO CATCHY!)
aLttP: Dark World theme (epic :D)
LA: Tal Tal Heights, Ballad of the wind fish is close second
OoT: Either Song of Storms or Gerudo...