Hmmm tough one really.
Will probably end up falling on Dishono(u)red so far. If judging by what it set out to do and what it accomplished. Very rich world does not stick any of its lore in your face at all. Was refreshing really. Although the NPCs suffered fromt the usual lack of depth problems, but oh well, no one is perfect.
My money were originally set on Mass Effect 3 before it was released but... I have had a change of heart. Besides Tuchanka and Rannoch, the arcs were too lazy and uninspired, the exploration and depth was none existent and it was a jack-of-all-trades that tried to be everything other games had done... instead of just running its own thing. Felt more like a linear action adventure game with weapon customization and a good/evil conversation system at times.
That and it relied on a lot of fanservice and almost no innovation. I am keeping the ending out of this whole judgement because that has resigned itself to the permanent archives of gaming history anyway, but is still not what the game as a whole should be judged by.
Dishonored did not really set itself up for much, but gives you a lot of variety, albeit a bit short and limited in some cases - it is still the best attempt at a stealth game I have seen in a long time. Now if these guys at Arkane Studios can just get together with Eidos Montreal and combine the rpg conversatios aspects and customization of Deus Ex: Human Revolution, and the stealth mechanic/combat, experience gain and morality system of Dishonored, I am sold, utterly and completely.