I've never really been addicted to a game like I was for Mass Effect 2. I couldn't stop playing, and when I did, I wanted to go back and play it again ASAP. I could never put down ocarina of time when I first played it, or twilight princess. Oblivion did that to me as well... I dunno. Most addicting game, well let's put it this way, I played Final Fantasy 13 for 12 hours straight, I've never done that before... So maybe that's the winner.