Hmmm that's a tough one. I like so many games for different reasons.
I'd say my favorite game series is a draw between the Half-life series on PC, and Gears of War series on the xBox 360 (tho my console is now long dead).
But, my favorite game at the moment when I'm not in the mood for Minecraft (and have a lot of free time) is Elite: Dangerous - a remoke of the old game Elite that I used to play on my Commadore Amiga back in the 90's. It was such an open and complex game back then, and the new version is pretty darn impressive and very well thought out (plus like minecraft, it's constantly being developed - coming to MAC and xBox soon).
But (again), I think the game I've spent the most time playing on to date must have been Gran Turismo 2 on the PS2 - I think I'm onlyabout 40% through the game, but have spent countless hours playing it (including a 12 hour endurance race). For it's time it had fantastic graphics, a huge variety of cars that all handled differently, and a large number of tracks. Plus it was the first racing game I'd played which used music that wasn't just Dance based - and it worked really well.
I'll stop now or my list will go on
