He's got the voice, and the build for it. You may never see his face, but I think Jeffrey Dean Morgan could handle it.
Runner Up: Karl Urbanalso RANKED
The starship AI that won the hearts of geeks everywhere, should be played by an actress that has done the same. Feisty and intelligent while also being vulnerable, it was a match made in heaven. Plus, if made right we'll get to see a weird CG body scan naked version of Evangeline as Cortana.also RANKED
Gunnery Seargent Buck
Why mess with a good thing. Let's include ODST Buck in the mox and cast the man who played him in Halo 3: ODST.also RANKED
Michael 'Mickey' Crespo and the voice of the grunts.
Alan Tudyk voiced marines in the origianl games and Mickey in ODST. I say bring him into the movie and also have him work a little over time providing voices for a grunt or two. His range is so diverse he could do double duty and no one would notice.
Sergeant Major Avery Johnson
The stogy smoking Sergeant Major is at the frontline of almost every major battle in Halo, and Elba has displayed the tenacity and sarcastic timing necessary to play the bad ass that gives Master Chief his marching orders.also RANKED
L The List