Right now, I'd say my favourite game is The Sims 3. It really changes every little while though, although it's usually an Elder Scrolls, Fallout, Sims or Battlefield game. I love playing the Sims for... some reason, I can't really put my finger on it. Elder Scrolls I love for its exploration and its lore, Fallout is much the same and I greatly enjoy the way most of the Battlefield games play: their guns feel great, they have large maps and powerful vehicles.
Played Sims 3 for something like 7 hours today and got my lowly fairy, police Sim from being about rank 3 in his career to maxing it out as a Dynamic DNA Profiler with 10 creativity, 10 logic, a vampire wife and immortality. Not too shabby.