Favorite Steam Game?

Hey,

As the first thread in here I think it would be appropriate to ask what is your favorite steam game, made with the valve engine.

Mine personally is Team Fortress 2 but I don't really play games on the valve engine so that would have to be my favorite one.

What is your favorite game?

I can't choose one :/

CS:S
CS:GO
Portal 2
L4D 2
Killing Floor

Well, i know that some of these aren't steam only, but...

I do personally REALLY like the Fallout series,
as do I also like Super Monday Night Combat (which i believe is steam only. note: SUPER monday night combat and Monday Night Combat are two different games. They are the same game initially yes, but they're builds are different and they don't play quite the same.)