The Dungeon Keeper games (including War for the Overworld, now that it's basically done): They're just so damn charming that I can't help but keep coming back to them just to tool around. Even just building a dungeon and watching your minions work, eat, sleep, train, collect wages, etc. is incredibly entertaining.
X-Com (EW): I don't even pretend to be that good at the game, but something about the atmosphere and set-up of this game just keeps drawing me back in over and over again. It's just so much fun to start out with a paltry force of rookie soldiers and a few fighter jets, and then over time and research, eventually be running around with power-armor wearing genetically engineered psychic super soldiers with plasma rifles, defending the earth with reverse engineered fighter UFOs that cruise at Mach 5.
Skyrim: There's just so many damn mods in addition to the insane amount of content, that it's never the same game twice.
Torchlight 2: Similar to Skyrim, there is a ton of mods for this game including lots of new character classes that I constantly find myself going back and playing it again.