I feel like some of you guys missed what the OP was talking about.
He means a game that when you first played it, you get this feeling of amazement and wonder, like you would get if you visited something like the pyramids. Something that feels almost unbelievable. I don't think he just meant a game you really like that got you into gaming or something.
2 Games have done this for me:
EarthBound, IMO one of the most underrated games of its time. The characters are so well developed, and you feel like you can relate to them so well. It's hard to explain the feeling I had when I first played through the game.
Metal Gear Solid 2/the Metal Gear Solid story, there is just something about this story that is so awe inspiring. Reading the extra background info included in the game and realizing connections to the previous games just floored me the first time playing through.
c&c and total annihilation did it for me .. untill i discovered baldurs gate 2 and it blew me away