The XCOM revival is the easy pick of the poll, but I'm so glad I'm not the first one in this thread to throw some much-needed weight behind the outstanding Mark of the Ninja. Those two are my true contenders.
All respect to those naming NSMBU, too. Its position was significantly strengthened by NSLU the following year, but even it launch it was easy to overlook its countless qualities and original contributions if you were stuck in the haze of NSMB fatigue or if you totally ignored its challenge mode and asymmetrical co-op. The pinnacle of the NSMB era, no question, and for all its conservatism, still my favourite Nintendo release in a sparse yet weirdly intriguing Nintendo year that brought us Kid Icarus: Uprising, Xenoblade (about which my sentiments are mixed), and the fascinating Nintendo Land.
Borderlands 2 is still the stock example I pull out today of a game that looked like the world's most horrible fit for me on paper, but turned out to be so much more than the sum of its parts that I still haven't gotten over the shock and surprise of how much I liked it. It might be my most-played FPS of any sort by now, and I recently fell right back into it.
Not sure what would round things out after those four (MotN, XCOM, NSMBU, BL2 in some order). I'm probably forgetting something important. I admired a lot about games like FTL, CK2, and The Walking Dead, but selectively, with caveats, and without ever being all that crazy about them. And Diablo III was a game I played and enjoyed more in its original state, RMAH and Inferno and all, even if it really was an objectively worse design than what it eventually became. Mists of Pandaria looks like one of WoW's highest points in retrospect, but it wasn't quite there yet at launch.
For all this, I still think of 2012 as the weakest year of the decade in my books, a title only contested by the tepid 2016, which had even less to offer me but which arguably hit higher highs.