My vote goes to Fallout. It simply has the richest story and setting to draw from--sure, a Gears movie would be epic, but it would more or less be a generic big-bdget sci-fi action movie. It would be fun to watch, but it can't compete with Fallout.
However, I do agree with you that Portal would be a good second choice. It may not translate to a full feature film, but I can see it becoming a short about Chell and GLaDOS's relationship (with a few really cool special effects thrown in). If they were to make this, I wouldn't want to see anyone GLaDOS portrayed by anyone other than Ellen McLain, who voiced the character in the game.
I think that all three of these could be viable films in some way (Fallout as the dramatic, post-apocalyptic one, Gears as the action movie with pretensions of emotion, and Portal as the artsy character piece), but Fallout is the one which I think would work best.