Destiny - The best gunplay I've ever experienced, and just generally excellent gameplay. It has plenty of flaws, but I managed to get several hundred hours of enjoyment out of it. That's several times more than any other game in my life.

I'd put Mario Kart 8, BotW, The Witcher 3, RDR2, Mass Effect 2 and Destiny 2 also up there.

The Witcher 3! Easy win for me! And that’s saying a lot as the decade includes God Of War, Uncharted 4, TLOU, Horizon: Zero Dawn, Dragon Age: Inquisition, and many more great games. But The Witcher 3 is a stand-alone classic!

Probably Zelda: Breath of the Wild for many reasons but mostly because of how they managed to make such a massive world so interesting and engaging to explore.

Galaxy 2 and MK8 are pretty close though.

demon souls probably, god of war 3

I feel it was a great game, but doesn't surpass uncharted 2 in my opinion, in terms of overall gameplay and set pieces. 

For me it is a very close race between Breath of the Wild and Dark Souls.  Not sure which one is better really.  I also like them for very similar reasons.  Both feel like they use old school sensibilities, design principles, and challenge.  But they also take all of these things and put it into a modern looking 3D game.

I guess I can say that Breath of the Wild is the game of the year for all Nintendo systems of the past decade and Dark Souls is the game of the year for all Sony systems.

Breath of the Wild.
Game revolutionised the open world genre and delivered on all fronts. It got me excited for Zelda again.

Horizon Zero Dawn

Runner ups:
God of War
The Witcher 3
The last of us

Breath of the Wild

Really drives home how long it's been when I remember that 2010 games were the same decade we're still in. 

I started off the 2010s having just gotten the Wii and playing stuff like Mario Galaxy 2, now I'm ending it with Witcher 3 on the Switch.