Building on the memorable themes from Halo and Halo 2, Composer Martin O'Donnell makes the finale of the Halo Trilogy even more memorable. The Soundtracks of Halo and Halo 2 are all memorable but I find Halo 3 to be the most enjoyable of them all. They give a fitting finale to the trilogy in a large majority of its songs. The main theme of Halo is also remixed quite a bit through the Soundtrack but always in a very enjoyable way. Most specifically the piano version of the main theme I like the most in the entire soundtrack.
Halo 3 - Main Theme (Piano Version)
Halo 3 - Halo Reborn
Halo 3 - Final Level Theme
Martin O'Donnell did fantastic work with the Halo franchise, helping to make the game's music as Epic as it's setting. But even better than the Halo 3 Soundtrack was the Halo Reach soundtrack. It did not feature the main theme of Halo very prominently at all in the soundtrack but built upon rather different themes. The soundtrack helped to show that the planet of "Reach" was a unique place, and that it should be fitted with music that would differentiate itself from the normal themes of Halo. Most of the music you hear in Reach is not present in Halo 1 to 3. It makes the game certainly feel more seperated from the trilogy and all of the music composed is really well done. Personally I enjoyed the Halo Reach Soundtrack more than the Halo 3 soundtrack.
Halo Reach - Overture
Halo Reach - Winter Contigency
Halo Reach - Tip of the Spear
Halo Reach - Epilogue