Tropico 4 - Tropico took me by surprise, I was hooked on it for about month non stop. I love when you fire up a game on a whim and bam! You have a new fave.
Fallout New Vegas - Obviously.
WoW - Pre Cataclysm I played that game hardcore. Never returned.
129 at present. But I'd say around 20 hours of that is just messing around with mods, technical problems, restarts etc. An awesome game though, and I still haven't touched the DLC yet. :)