I think it varies from game to game, as each one has at least a handful of tracks that are all equal in terms of beauty relative to each other. I guess it depends on my mood. I adore the soundtrack to Reach, and I love how Halo 2 has a hard rock feel to its most epic tracks, but I also find Combat Evolved to be the most consistent in tone. Halo 3 is pretty good overall, and I do like the music from the Anniversary editions of HCE and H2, although HCEA's score is a mixed bag. Some tracks are fantastic, while others feel wrong because of horrible timing in certain places, like Brothers In Arms or Dust And Echoes, which were two of my favorite tracks in Combat Evolved. Thankfully, the people working on Halo 2 Anniversary seemed to have learned from the mistakes of HCEA. In fact, I would probably rank H2A as my favorite soundtrack if not for one... teeny... tiny... little... issue.
Can you guess what it is?
Here's a clue.
"You might want to sit this one out." - Cortana, Mausoleum fight, Gravemind.
The lack of Blow Me Away in the Anniversary Edition's soundtrack. The music they replaced it with is decent for what it is, but... it just doesn't feel right without Blow Me Away in the background. The fact that I have to switch back to Classic Graphics to hear that track makes it all the more frustrating for me, because the rest of the H2A soundtrack is so superbly orchestrated and conducted.
So, in the end, I would have to say that it's a tie between all of the classic and Anniversary games. If it weren't for the lack of Blow Me Away, however, Halo 2 Anniversary would be my number one favorite.