"Xion's Theme" (also known as "Musique pour la tristesse de Xion", literally "Music for the sadness of Xion") definitely counts, containing a melancholy variation on Kairi's theme that is quite fitting for the character.
Then there's "Critical Drive", one of the final battle themes in the game. While the KH2 version, "Deep Drive", was all right, the 358/2 Days version really brings out Roxas's and the player's drive to beat the mission.
The Kingdom Hearts 358/2 Days "Sacred Moon" remix named "Mystic Moon", heard as Roxas escapes the castle. While neither truly original, nor action packed, it communicates tension, fear, and uncertainty in Roxas' leaving of the Organization.
The opening song, "Sanctuary". While it made its first appearance in Kingdom Hearts II, the lyrics really fit with the Nobodies, especially Roxas.
"Fight and Away" is a fantastic remix of the two boss themes, Struggle Away and Fight for my Friends, from Chain of Memories and makes any fight ten times more epic.
The very melancholic yet beautiful "Missing You", debuts in Kingdom Hearts II but fits the theme and sadness in Days much better.