I'mma count Skyrim cause modding didn't really come into fruition until this year, and mods made the game a whole new experience.
Other than that, ... Kingdoms of Amalur.
I'd also say Skyrim because of the mods. I'd find a new mod then play around with it for hours, swap ones in and out, start new characters, etc... I also played a lot of Amalur Reckoning. I tend to over-level so I'd do practically every quest I came across. The loot I used was all crafted stuff, so gathering mats for new items was fun.
After that, Borderlands 2. Like Amalur and Skyrim, I did everything and created multiple characters to level up.