To Zanarkand and A Fleeting Dream are probably my two favorite tracks in the series, and most remixes of those two (or rather Suteki da ne) are pretty damn great. Special mention to Sight of Spira which really nails the predominant atmosphere of the game's smaller settlements.

Also shoutouts to A Contest of Aeons, Assault, Silence Before The Storm, Rikku's Theme, Fight With Seymour, the Normal Battle theme, Wandering Flame and of course Otherworld.