So many memorable OST's in here, the nostalgia is real!
There's so many soundtracks that are very memorable to me, but the one's that stood out for me the most are the Warthog Run track at the end of Halo 3, and the Imperial March theme from Star Wars. They are both iconic to their verses, and just fills you with adrenaline when they start playing, it's brilliant. As for the Imperial March? I'm a sucker for the Galactic Empire, I absolutely love them so hearing that theme play when a army of Imperials attack a place just fits so well.