For me The Witcher 3, Borderlands 2 and Skyrim are all worthy contenders. Nobody would bat an eyelid if I picked anyone of them to be my game of the Decade.
Other games that I would rate as right up there with the best would be any of the Tomb Raider games, Dragon Age Origins, Metro 2033, GTA V, Guild Wars 2 and Civilisation V.
But, I have narrowed down my game of the decade to the following:-
Faster than Light.
Stalker: Call of Pripyat.
I can't really choose between them.