This is a pretty easy one, for me it's gotta be the Littleroot Town theme. No other song, video game or otherwise, is as calming or nostalgic to me, not even the actual pokemon theme song. I was really scared ORAS was gonna ruin this theme the same way HGSS ruined the New Bark Town theme but they did it so much justice I got hella emotional the first time it started playing in-game.
