The Event Music Stages, particularly the one for [insert your favorite game here]. They will make you want to go back and play the games again, and you might even need a tissue or two before you finish the stage.
Nobuo Uematsu's quote on the main article.
For Chrono Trigger fans, one of the DLC songs is "Wind Song," the theme that plays in the 600 A.D. overworld. If you look closely, not only can you see Frog looking out over Zenan Bridge, but also Robo in his efforts to help replant the forest.