My suggestions:
1: Peter Jackson. Original planned-to-be director of the alleged Halo movie, laat at what the guy did with recent movies like: King Kong and Lord of the Rings. He will always remain my first pick to direct this movie.
2: Guillermo del Toro. When I heard Guillermo might have been attached to the movie as the director, I was really looking forward to the movie. It's such a shame he rejected to do Hellboy 2 (Which was a decent movie nonetheless) Brilliant director, Pan's Labyrinth and Hellboy were really good fantasy movies. I had no doubt he would do fine with a Sci-Fi setting as well, alas we will never find out.
3: Roland Emmerich. This guy's movies always heavily depend on A LOT of CGI (Computer-Generated-Images) and that's exactly what a Halo movie would need. I'll take Independence Day, The Day After Tomorrow and the upcomming 2012 as an example. Halo would need to be big-budget and would need a lot of cgi. This guy could get that job done.