Favorites in order, best to worst (IMO)
Halo 2 (I dunno about you, but I loved most of the tracks on the H2 OST, specifically Mjolnir Mix, the Mombasa Suite, and not to mention the inclusion of tracks from Incubus and Breaking Benjamin was a nice touch as well. Also, I love Earth City. All hail the Halo 2 Scarab.)
Halo 3 ODST (Somber, sad, hopeful. 3 words that very accurately describe the game and its soundtrack. I love how it's centered around piano music, which captures the environment of the game in a way that beats Halo CE, in my opinion.)
Halo CE (Originals hold up very well when compared to the others, specifically the original Truth and Recon theme, Rock Anthem, and Under Cover/Night. Really matched the environment and the tone of the game well. Shout out to SPV3's soundtrack, which did an even better job with the soundtrack than CEA did.)
Halo Reach (Had a lot of good tracks in it, had the same effect that ODST did but on a bigger scale. Only reason it's under ODST and even CE is because not that many tracks stood out. Really, the only tracks that did were Tip of the Spear and the intro to Lone Wolf.)
Halo Wars (Very angelic feeling, mixed in with heated tracks to match the time the first shot is launched on an enemy base. I like it. Spirit of Fire and Bad Here Day were new and welcome tracks with new tones to delight our senses of sound. Reason why it's low is because some of the tracks can be a bit repetitive.)
Halo 3 (Really, the only reason I like it is because of the Warthog Run and One Final Effort. That's it. Everything else kinda just mingles into the background for me.)
Halo 4 (A depart from the traditional sounds that wasn't that much appreciated. Hell, 117 is the most liked track by everyone else and even I don't like it. There's just nothing notable with the Halo 4 OST.)
Halo 5 (One word. Repetitive. Very. VERY. REPETITIVE. Really, the only thing I like hearing is The Trials, and that's it. Everything else just gets muted.)