Starcraft would be a pretty cool movie if it turn out to be cheesy like Starship Troopers (which was actually quite entertaining :P )
Splinter Cell could easily be made into a movie. The game is practically made like a movie.
I'd love to see Halo made into a film. I've heard many rumours about it, but the main thing is that if they are gonna make it into a movie then they should do it right! And not rely on the Halo name to sell the film.