To me, as with many others, it's close to impossible to choose one song. I think all of the Halo soundtrack is awesome, and I've always been under the impression that it's half the reason why the games are so great. Martin O'Donnell is an utter genius.
I guess since I have to pick though, here are some that have been played a fair few times.
A walk in the woods!
Peril, Under cover of night, In Amber Clad, High Charity Suite and Unforgotten.
I could keep going.