Horizon Zero Dawn. Its rare for me to want to actually finish a big open world game of this type, but I really enjoyed the world of HZD and I wanted to bring it to completion. I really enjoyed Aloy as a protagonist, the environment design, the side stories just everything about this game kept me coming back.
Honorary Mentions:
Bloodborne: It has easily become my favorite souls game. I finished it this year, but I played about a quarter of it the year of release which is why I'm not fully counting it. Otherwise it would be my Backlog GOTY.
Until Dawn: I played through it with a friend and we each chose half the cast to control/roleplay as and we had an absolute blast. Not sure how I would've felt about it solo, but together it was great.
Life is Strange: Before the Storm: I had no real hopes for this prequel to one of my favorite games (Life is Strange was my backlog GOTY for 2016). I had been turned away by the announcement of it since I didn't really feel it was necessary to retread on the past of characters who had already been fully explored in LiS. However, Before the Storm turned out to be a really great romp through Arcadia Bay.
Okami HD: I have not finished this one so I'm not fully counting it, but I'm fully loving it so far. Great music, great aesthetic, great game.