Halo Theme (Halo)
In my humble opinion Halo's grand orchestral theme is better than the game itself: probably the only genuine classic game score since Hollywood-style compositions came to replace simple and repetitive electronic ditties. Composers Martin O'Donnell and Michael Salvatori really outdid themselves with this.
Original track: http://www.youtube.com/watch?v=e6JK3fhYE1w Give that the above is clearly performed by a big orchestra, it's amazing that the cover below captures the same level of grandeur with just three musicians. It's pretty fantastic.
Awesome cover: by
corporeal http://www.youtube.com/watch?v=VLt5_ME_2_M