Personally, I´m in love with the “Roll Call” song from Halo 3, but I also think that “Never Forget”, “A Walk in the Woods” and “117” are one of the best among all the Halo OSTs.
Mine is Halo Reach Extra OST Never Surrender
117,Arrival and Halo.
If I had to name a favorite, Under Cover of Night probably takes that place for me.
it tends to vary depending on the situation or mood, and I have trouble choosing just one so I’m just gonna say somewhere along the lines of unforgotten, ambient wonder, cratered, peril, ODST overture, and in amber clad