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 Urban
#593 on The Best Actors in Film History
#223 on The Coolest Actors Eversee more on Jeffrey Dean Morgan
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 Rankedsee more on Evangeline Lilly
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.
#159 on The Best Actors Working Today
#605 on The Best Actors in Film Historysee more on Nathan Fillion
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.
Also Rankedsee more on Alan Tudyk
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 Rankedsee more on Idris Elba
If you can't tell by now, I want a Halo movie to be a blance between ODST and Halo. ODST had dynamic characters that I'd love to see again, and the casting as spot on, so why not? Let's include geek favorite Adam Baldwin as his ODST Marine, Dutch.
#131 on Famous Tall Men Over 6'2
#15 on Famous People Named Adamsee more on Adam Baldwin
Commander Miranda Keyes
The Commander of the ship "In Amber Clad" Keyes is pivotal in the fight against the covenant. She's intimidating and commands respect.
Runner Up: Katee Sackhoff
#249 on The Best Actresses Working Todaysee more on Adrianne Palicki
The Prophet of Regret
The chameleon actor would be perfect to play the Prophet that tries to lead the covenant on a suicide mission into another universe by activating all of the Halos. Serkis ability to shine through, even when he's not technically on screen makes him perfect.
#314 on The Best Actors in Film Historysee more on Andy Serkis