For me it's Jet Set Radio Future. I played and replayed it many, many times. I still get the urge to play it. My first copy was on that dual disc that came with the original xbox and so my 360 wouldn't play it. I called several different game stores to see if they had a copy of the single game. Ended up traveling to a store in another state to get it: a disc without it's case! I logged a great many hours into that copy, that's for sure. I think it's just because the whole game is so pretty and has a fun soundtrack.
It was the first game that I logged more than 10 hours into. Mostly, this was because it was on the first "real" console we had gotten (xbox when it first came out) and games were very expensive, too expensive to buy a ton of new ones for several months after we got it.
We use to have a N64, and I would play Zelda OoT, but I was a chicken and couldn't do the whole action-adventure genre. It was just too scary for me. I've gone back to it from time to time since I've gotten the Wii. Very good game, I like it more with each play through.
Lately (as in the past few years) it's been Aquaria. I love that game! So much replay value, for me at least. I think it's the easy controls and mesmerizing sound track. Kinda a theme with the games I like.