It's an epic battle between a Super Mario and Fallout 3 but I gotta go with Fallout 3. At the moment at least. I do enjoy other games but I'm so hooked on Fallout at the moment. I just love rampaging through the Wasteland, killing anything that moves and get killed by something bigger just because it moved.
Second one on my list, most likely jumping to nr. 1 in a few weeks is TF2. You DO get bored of it, I know I did. But I took a break, played World of Warcraft, beat my head against the wall for achieving nothing and played a bit of TF2 again after a few months(?). The result of that is I noticed it wasn't boresome anymore, and imma keep on playin'. =P