Never Forget
From Halopedia, the Halo wiki
<video name="Never_Forget" width="250" align="right"/> Never Forget is a song included in the Halo 3: Original Soundtrack. It is based heavily off of the Halo 2 song, Unforgotten, with the beginnings and ends of both songs being very similar.
It starts with the Unforgotten theme played with strings, but four keys lower, giving it a more solemn mood. The motif is then repeated with choir. Choir then takes the melody, with a piano accompaniment. The song then swells to a definite upturn, switching to a major key. Almost immediately, it switches back to a minor key. Piano then takes the melody, playing more of the Unforgotten theme. With careful ears a French Horn background melody can be heard. The last minute of the song is exactly the same as Unforgotten.
Contrary to popular belief, Unforgotten is played in the main menu of Halo 3, not Never Forget. On Halo 3: Mythic, however, Never Forget can be heard on the main menu.
It runs for 3:06.