I'll go by my reaction at the time I played it.
Mario 64, I think I spent 2 hours in the toy's r' us playing the demo, it was amazing.
Ocirina of Time also amazing.
When the Gamecube came out I was blown away by Rogue Squadron.
Dreamcast days Shenmue was amazing, I just wanted to live in that little town, the attention to detail was amazing.
And this gen believe is or not what has impressed me the most is Just Cause for the 360. Honestly I've walked around the beach just to enjoy the scenary in that game. And it has actually relaxed me. I call the game a mini vacation of couse thats when I"m not taking out the entire government. This video says it all:
Crysis though has been looking quite good, but I don't like to count a game that isn't even out yet.