For me it has to be two tracks, and those tracks are "The Last Array" for its incredible brass! And the other track is "From the Union". I gotta say when fighting the final progeny that track just makes it all too thrilling, even though the boss can be easy.
EDIT: Feel free to post other game sound track pieces if you like them. A good example is "Those who get in our way" in Xenoblade Chronicles. Awesome track
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Last Array rocks, also the track that plays during Eye of the Gatelord, the name escapes me right now, and the Cabal Stomp, lol, all of it! The music is half this game for me, my only emotional connection paired with the game's visuals, good lord what is Bungie going to do without Marty... I'm guessing random generic epic tracks.