That Nier music! Holy crow. I don't usually dig that kind of soundtrack but Nier's is so good.
Okay, I'm gonna do as you say and just pick one game: Spelunky. *yes* it's the enhanced port of a game released in 2008, but this is the definitive edition and the one that almost everyone played. I probably put 200 hours into Spelunky. I love it. (If you don't allow Spelunky then Rogue Legacy is my second choice. It was kind of a crappy year for games.)