Unforgotten
From Halopedia, the Halo wiki
Unforgotten is a song included in the Halo 2 Original Soundtrack. The first section of the song is included as one of the subsequent pieces in the main menu of Halo 2. The remake of this piece, Never Forget, is heard exclusively on the Halo 3 Original Soundtrack, as Unforgotten is played in the main menu of Halo 3, contrary to popular belief that the song is Never Forget. It is the only piece to be played on the main menu of two games (with the exception of the Gregorian Chant). A remix of this song is also played in the menu of Halo Legends. Unforgotten is often stated to be the most emotional piece in the series. What the name of this piece is actually referring to is unknown, but it could either be referring to the Master Chief, the Marines who died in the Human-Covenant War, or both.