It seems as though a lot of people are assuming movies based on these games will be direct translations... Comments about TF2 being filled with point and intelligence captures with no story, HL and Portal ones containing mute protagonists etc.
I understand a lot of those suggestions are jokes, but Valve has proven themselves more than capable of forming narrative. Even with mute main characters.
Hell, look at the most recent TF2 update with a 21 page comic detailing the nade/rocket spam update and what brought it about. Meet The Spy is also a great example of their filmography skills, along with both L4D intros, moreso the second one.
People saying Portal would likely be a good movie are probably the most wrong, since very little actually happens in the game, it would be difficult stretching that out into a movie. It would be nice to see a little more background to Aperture Science, but the game already portrayed enough. All the movie would bring is a scene of GLaDOS killing everyone, then a load of portal jumping with creepy synthesised voices floating around all over the place.
A Half Life movie would be terrible purely because fanboys will hate it if it's different to the games, and hate it if it's the same as the games, so there's no pleasing them. Like any game based movie, no matter what they try, fans would cause it to fail at the box office.
A Left 4 Dead movie could work, but probably better with a new set of characters and locales. Using some from the games wouldn't work since most people who would watch the movie have already spent hours playing as them.
A TF2 movie is probably the best idea. It's already been hinted at (along with comics, TV/web show etc.) on the TF2 blog, and with things like the Meet the Team vids showing the team's skills at creating animation, it would definitely look the best. There is also the fact that TF2 as a game has absolutely no story, besides this supposed history between the blue soldier and red demo, which would help it make a much better movie.
A completely clean slate for Valve to work in character histories, expand on the world that is Team Fortress and this evergoing war between RED and BLU which, to the best of my knowledge, seems to have absolutely no effect on the world itself. It's like a never ending Unreal Tournament. Just a bunch of middle aged mercinaries pitched in constant battle, both controled by the same omniscient watcher, shows a lot of promise as a movie or similar media.