Valve is only really at the point of almost monopolizing steam...
Outside of steam they don't have much of an effect.
Portal 2 on the PS3/360 is great but there are a lot of better titles for consoles out there.
Also, "finest games in history" is not defined, everyone will give a different answer if you asked them.
You do realize Steam has like 75% of the market share in PC game distribution, right? That's pretty much a monopoly.
Also, saying there are "a lot of better titles" than Portal 2 is downright wrong. Portal 2 is a masterpiece. Difficulty curve is perfect, puzzles are carefully crafted, story and environment are some of the best presented, and all the mechanics are good. It's literally a perfect game and has, in fact, made it to the top of many "best games ever" lists. Same with Portal.
Just across the board Valve have made the best games and the best innovations in gaming. They popularized digital distribution and set the model on how it should work. They were able to turn TF2, a game that was already incredibly profitable for them, into an even more profitable game 4 years after its release, without sacrificing anything. They've been able to make incredibly unique games using the same engine and all of them still first person for years. They not only work with gamers but for gamers.
Regarldess of your opinion, Valve games are the finest. That's a fact.