Well out of respect (and boredom) im going to include all the candidates
Skyrim, GTA V, Withcer 3, Bloodborne, Zelda BOTW, Mass Effect 2, Dark Souls, Fallout New Vegas, fallout 4, Assassins Creed Black Flag, Tomb Raider, Bioshock Infinite, God of War 2018, MGS 4+5, Borderlands 2, Far Cry 3, Super Mario Galaxy 2, Last of Us, Uncharted 4, Mario Odyssey, Red Dead Redemption 1+2, Heavy Rain, Spiderman 2018, Portal 2, Batman Arkham City, Deus Ex Human Revolution, Gears 3, LBP 2, Dishonored, Titanfall 2, DOOM 2016, Smash Bros Ultimate, Forza Horizon 4, Divinity 2, Bayonetta 2, Overwatch, Black ops 2 (best cod this decade), Pokemon X and Y, Shadow of Morder, Nier Automata, Horizon Zero Dawn, Monster Hunter World, and Persona 5.
Obviously I havent played them all (just a handful) but this list seems to be the general consensus based on meta and public reception for best games this decade
Buuuuut if I had to go with one personally...
Witcher 3- Best game I've played this decade hands down with Skyrim, RDR2, and GTAV following closely
2019 Predictions- Kingdom hearts 3, Res Evil 2 Re, Last of Us 2, Outer Worlds, Cyberpunk 2077 (If released in 2019)