Witcher 3 by far. It's been a while since a game managed to amaze me with what is possible and/or made me cry like a little bitch.
Thanks to my habit of only picking up GOTY editions when on sale, I haven't played much else. SC LotV was/is pretty good, almost unexpectedly so, but it still didn't blow my mind like W3 did. Thanks to early access and all sorts of DLC, the release list gets a lot blurry. Does new client or huge update count ? In terms of time spent, it's Dota 2 for me, no contest and maybe Path of Exile thanks to the 2.0 patch. Diablo 3 also always catches my interest with a patch and a new season. I wonder how many people who actually played W3 don't have it in their own personal GOTY list.