Xagor's theme has to be one of the best BGMs of the series, and that's saying something with a series that had "Let It Begin" as a leitmotif. Heck, the entire last battle has awesome sounds — starting as you're fighting "Sol" (the local God analogue), as he desperately tries to get you to do as much damage to him as you can before Xagor (the local Eldritch Abomination) tears him apart from the inside out to get to you. Starts here, with Sol's slow, sad theme, switching into Xagor's theme around 36 seconds in. Pure awesome.
Feldschlacht III is the best regular battle theme in the game, with its piano and percussion giving the song a unique tone to it and the battles it plays during a very atmospheric and moody tone to them, while Feldschlacht II uses an organ sound to great effect.
This game is an interesting case: it's the second SaGa game with a soundtrack composed by Masashi Hamauzu instead of Kenji Ito, after SaGa Frontier 2. The OST is divided into two distinct halves: the first half is more traditional sounding orchestral tracks, while the second half is mostly comprised of a lot of experimental electronica. Both halves are equally awesome. Unfortunately, the score is often overlooked due to the... dividing quality of the game itself.
The OVERTURE, a very energetic song. Perfect for to play for starting on an EPIC JOURNEY.
The numerous battle themes, with four on Disc 1 and a lot more on Disc 2.
Battle 1 is the perfect music for the first battles of the game. Just the beat and the violin in particular really get you pumped up.
Battle Theme IV: A tango-themed battle theme, complete with foot stomping. Attack your enemies in a way that would make Astor Piazolla proud!
The final boss theme, BT "ultimate" manages to evoke a wide range of emotions, 'awe' being one of the most prominent ones. The final boss is fought in space. The song begins with some strings and atmospheric synth sounds. A heavy techno beat is added, with digital sounding synths in the background. And then comes the trumpet melody, which is amazing. It really uses the word "awesome" with its original meaning.
Enigmatic Scheme is a great, goofy, bouncy theme like few things Masashi Hamauzu has written. What sets Enigmatic Scheme apart is the slightly sinister tone. It sounds like it could be the Affably Evil theme song.
Other bouncy, cheerful themes of his include Judy's theme (mistakenly labeled as "Feel Uneasy About the Wonders" on the official OST release) and a Brief Respite from FFXIII.