It's hard to say really, I enjoyed quite a few games this year.
Overall, I'd still give it to Nier as I know there won't be anything as weirdly creative and artistic till Taro's next work. And the damn soundtrack... Shame that many people didn't bother to play the game through and through or even try it but whatever.
Most fun game and also very unique by today's standards? Yakuza 0, easily. Though I played that last year and I'm now playing 6 as I have both in Japanese.
Other than that, AC: Origins has a jaw-dropping map (probably the best I've seen) which I enjoyed tremendously and is overall a good game.
WRC 7, GT Sport and Forza 7 are my favourite racers this year, don't think I can pick the best one.
My most played game this year is Tekken 7 and it certainly gets The Most Frustrating But Also The Most Rewarding Game of The Year award.
There are some good games like Horizon, Gravity Rush 2 and a couple of others which I can't fully judge as I haven't made enough progress in them yet (and probably won't for at least a couple of months) so I'll leave it at that.