To me, it's a tie between Morrowind and Skyrim. I simply can't decide. Both are composed perfectly to the games and wake up some memories.

Morrowind's, if only because the theme song is designed so in tun with the game, what with the drumbeat being Lorkhan's heartbeat and all.

If the "snowfall" tune from Daggerfall was a theme song, it would be it. Or the "rain" one. Or maybe the "castle" one. Any song from Daggerfall to be honest. Yes, even the shop music

Morrowind > Oblivion > Skyrim but they're all pretty awesome.