I think the most successful video game movies would have to be ones that build on the premise of a game rather than the existing plot. Personally, I would find a movie simply recounting a plot that I already knew boring at best. Books hold a lot of potential for movies because audios and visuals can bring something very new to a story, but most video games these days already have incredible visual components and amazing voice actors. Live-action would seem to add little to what's already there, and I think most audience members would just get caught up in all the details that the movie didn't get right.
I think this effectively rules out games like Mass Effect, because even though there is a huge universe that exists outside of the plot, it's so closely tethered to the character of Shepard and his crew that any Mass Effect movie without them would feel odd. Instead, I'd agree that a game like Portal could become a great movie if they explored the background of Aperture Science, or built a new plot based on the portal gun technology. Sandbox games like the Elder Scrolls or Fallout series would also do nicely, since those games emphasize the actual world, which would then create a lot of material for a movie production team to work with. One notable exception I'd bring to all of this is the Assassin's Creed series. On the one hand, each individual title has a distinct plot, but subsequent titles have also changed the main character -- from Altair, to Ezio, to Connor. Given that, I think the movie can definitely create a new protagonist that would remain consistent to the overall theme of the franchise, and all of history would be open to them (because at this point, I don't think anyone's trying to retain any sort of realism or logic to Desmond's ancestry anymore).