I would say Portal with Half-Life 2 coming in at a close second.
I bought Portal without knowing almost anything about the game and was just blown away by it. While Portal 2 might be a better game, the impact the first one had on me was amazing.
I take the Half life 2 games as a whole, so its pretty easy decision. Personally I think playing Portal 2 is one of the greatest experiences I have ever had and is probably the first game I will recommend anyone who talks shit about video games, but find it hard to qualify it as one of the greatest games ever (I found both protal games incredibly easy). I think Half Life 2 is a way better game. Half life 1 is very close to Half life 2, but I think the face animation and story of the second + gameplay and the cool stuff you do in Episode 2 makes it better than the original.
Also Dota 2 is fantastic, its like Dota but with modern UI and Valve's charm (the little humor with the character speeches and animations are fantastic)
Episode 2, no question. I'm not saying that Half-Life didn't have a deeper impact on me because it had a seemingly astounding scope for its time, and Half-Life 2 was an equally amazing step forward, but Episode 2 is a culmination of the evolving game design philosophy at Valve. It's a showcase for what they've learned, and an example of their best work.
im probably alone in thinking TF2 isn't that great. like its not a bad game, it just feels dumb. ya know? i've loved everything else valve has done. i could never play a match or two of TF2 before thinking "meh, this is dumb, im going to do something else". i just find people who play it for hours and obsess over the hats and stuff to be crazyyy