There are many games that I'd love to see on the big screen, but what I really want to see adapted into a film is Myst. I've played through almost every Myst game and it could be an amazing movie.
I'd say Final Fantasy 6 needs a good movie, or better yet an animated series, also FF 13 Versus which is coming out soon, is like a playable movie it's that amazing.
Professor Layton already has a movie released in Japan... I think the story takes place before the first 3 games (Mysterious Village is game 4 mind you, Diabolical Box is game 5, and the Last Time Travel is game 6, like Star Wars...) AND Professor Layton is due to team up with Phoenix Wright in either a game or movie coming soon.
Dead Space. They already have a badass Isaac Clarke costume, so why not? They even have a life-size Plazma Cutter prop. They could make special Necromorphs just for the movie. A live-action movie would do the series justice, in my opinion.
I forgot to add something.
Portal. GLaDOS would be epic to see on the big screen. Some fans of the game have already made a life-size replica of the Portal gun, and an Aperture Science jumpsuit would be pretty easy to make, including the leg augmentations.
Half Life. The games are awesome. The story is awesome. Gordon Freeman is awesome. Some fans have created Combine costumes that look pretty close to the in-game models, along with Stunsticks. A movie for either one would be nice.