Deus Ex. I was 14, didn't really play or like first person shooters and only played it because it was in the pile of old games someone essentially dropped off on me. But I understood rather quickly that this game was something special, especially with the player's choices actually making a difference. I remember a moment when I was playing around with cheats and at one point spawned a Paul Denton next to his dead body in the lab under UNATCO just to be funny and when he started talking to me as if he hadn't died, a quick online search revealed that I didn't even know how deep your influence truly went.

I still don't really like or play first person shooters, but that's now mostly because Deus Ex has set my standards very, very high!