The Half Life series would be a GREAT movie! So much potential!
The first movie could start with Gordon in Black Mesa, and the second can continue with him in a camp just like in the games. Half Life in my opinion has the most potential to be a movie compared to ANY other game out there.
When wondering what game could be a movie you cannot think about game-play. A game could be awesome but it needs a story. A good solid story that people can love.