I would have to try and choose between Red Dead Redemption and Deus Ex: Human Revolution. I know purely on time spent, I put several hours into Skyrim, Fallout 3, and Fallout: New Vegas. I know Dead Space is among my favorite game franchises. I liked to play as Batman, and feel awesome taking down a room full of criminals; not to mention the fact that I was able to play as Joker, Catwoman, Robin, and Nightwing with some of the games.
All that aside, I had the most fun and the most hours of enjoyment playing Red Dead Redemption and Deus Ex: Human Revolution. Those two games never made me feel like I was just wasting time to gain experience, and neither game had a moment where I didn't feel like I was in the old west or in a near futuristic society. Both games really rewarded exploration and nothing you did was really time wasted (all exploration in Deus Ex: HR showed new ways to get through a level or new weapons and story information, while all exploration in Red Dead Redemption just allowed me to explore the Old West more and keep emerging myself in the world. In the end though, Red Dead Redemption takes the favorite spot just because John Marston is one of the best, most flushed out videogame characters I have ever played as.