Fleet admiral
From Halopedia, the Halo wiki
Fleet Admiral is the highest commissioned officer rank of the UNSC Navy, with promotion to this rank restricted to times of war.
As there is no equivalent in the UNSC Marine Corps, Fleet Admirals outrank all Marine Officers (as well as most Navy Officers).
Fleet Admirals typially lead battlegroups charged with the defence of key UNSC planets, such as Earth.
