It's either Bioshock: Infinite or Metro: Last Light. Both had such great atmosphere, stories, characters and gameplay that judging between the two is pretty much impossible for me, especially because at their cores they're such vastly different games. Both were absolutely delightful and the fact that you can go back and play through the entirety of both in different ways is just the icing on the cake.
Honorable mention goes to the Dishonored dlc: Knife of Dunwall and Brigmore Witches for A). being DLC and thus not full games worthy of GOTY and B). because I don't really feel that they needed to be released separate of each other. Otherwise, the characters were, generally, much more interesting in the add-on than they were in the main game and powers like pull and summon assassin really added a lot of options to the game, that's not including the new equipment. But what really got me was the new Blink that stops time as well; that damn near changed how you play the game. Who could have guessed that such a simple addition could change so much!
On the opposite end of the spectrum for me: the worst game definitely has to go to the cynical, contradictory mess that is The Last of Us. I was actually looking forward to this, which is saying a lot because I strongly dislike the rest of Naughty Dogs work, but damn, playing this game made me so angry. In a way it's fitting that I thought this game was terrible while I loved Infinite and Last Light because in many ways TLOU felt like a combination of the two just third person. It's amazing to me that Naughty Dog get's the acclaim that it does when all it really does it make games that are, admittedly, presented extremely well but with gameplay that kind of, more or less, just works. To me The last of Us was no different.