valve has a way of bringing originality into genres that have been remade so many times, and being totally sarcastic and taking itself seriously with a deeper message at the same time. valve has also created the pc gaming market into what it is today, and unlike other developers in their field they are able to continually make great games while managing the entire pc game market with steam, and working to expand the gaming market to mac.
bungie has created themselves into a total legacy and have corrected themselves from doing what other developers have done wrong, and ending the halo's story at the beginning with reach was a great idea.
pandemic studios is long gone- but they were able to produce amazing games while they were still there. like valve, they were able to incorporate unfamiliar ground into creative wasteland game genres, if they werent downsized by EA like they were they might be on the mainstream market right now.
bethesda... as of today they are an amazing source of originality because of their ability to push all of their effort into one game at a time- a quality that large companies lack. after the success of fallout 3 I am hopefull that they will maintain that as they gain popularity.
out of all of them valve has the most potential because almost all pc manufacturers rely on them, and they are showing that instead of rereleasing a game without reason, they wait until they have an innovative idea that will make the game better. I am not saying that bungie does not show the same quality, I expect great things from them too, same with bethesda. with news of mercs inc. a year back maybe pandemic's creative designers will get lucky and we will see more out of them, considering EA backs them up with all the money they would otherwise be pouring into uncreative crap like metal of honor.