David Scully
From Halopedia, the Halo wiki
David Scully is the voice actor for Sergeant Avery J. Johnson in Halo: Combat Evolved, Halo 2, and Halo 3. He also voiced many of the Elites. His other credits include Mechassault 2: Lone Wolf and Douglas Holiday in F.E.A.R. (First Encounter Assault Recon). Interestingly enough, Scully has also done a voice-over with Jen Taylor in the PC game Aliens Versus Predator 2.