My favorite steam game would have to be Counter Strike: Source. Only because what got me into steam back in the late 90's. Counter Strike made a name for steam. Without Counter-Strike I'm not sure where Valve would be today IMO.
Portal, it has a great storyline, and it's fun overall.
My favourite steam game is CS:S
Half-Life 2, Portal, and Team Fortress 2. Ive only played at my friends house. I dont have steam myself.
HL2, GMOD DarkRP, Portal.
I would have to say GMOD. It allows the player to do stuff never really thought of before it.
Torchlight is fun :-]
Good diablo 2 clone
I would have to say counter strike is my favorite steam game out there specifically CSS because it has so many different type of game play from mazes to races as well as the normal shootem up gameplay
I've heard great things about Team Fortress 2.
Only played it once at a friends, it was alright.