By the power of truth, I, while living, have conquered the universe.
Halo 2, ODST and Wars were my favourite in terms of soundtrack.
Halo 2 was the most varied. We had tracks like In Amber Clad which built up a beautiful atmosphere and is the track that defines the Halo universe IMO. Anceint Machine was amazing too, every time it started playing on Sacred Icon/Quarantine Zone I'd feel the tension being built up as I race to the next piston whilst a horde of Flood and Sentinels chase me.
ODST was nothing short of divinity for its music. I could go through each individual track and write a novel on how stunning the noir-esque atmosphere is. I'll try to do it justice in a few sentences.
Neon Night, Skyline, No Stone Unturned - all the tracks are fantastic. Marty outdid himself, the soothing piano tunes playing whilst scouring a city aflame was the most visceral Halo experience I've had since Halo 2. The guitar and sax solos need no explanation of how awesome they are.
Halo Wars was my third choice because... well, it was great. Better, actually. Rippy proved that you don't have to be Marty to produce genuinely good music for Halo, I always get goosebumps every time I watch the Judgement Day cutscene because of the choir as the Forerunner Dreadnaughts awaken from their 100,000 year long slumber.