I thought about what criteria I'd use to judge the best game, but then I decided that I didn't want to think about criteria, so I'm just going to say what were my favorite.
I was tempted to agree with Mario64, and right now the game I play the most is Team Fortress 2.
But my favorite has to be a series of DOS Adventure/RPG games called Quest For Glory. (And if you're making me choose one, I'll choose the remake of the first one.) It was just fun. Not really linear, tons of cheesy jokes and pop-culture references woven in, an interesting story, and beautiful music. I played it growing up and I'm sure it had an impact on me.