For long time enjoyment: Splatoon 2
For 1 time play: Fire Emblem Awakening

That's hard to say. There's a lot of games to go through to pick a definitive game. Ones that come to mind are:

Breath of the Wild
Mario Odyssey
Xenoblade Chronicles 2
Splatoon

I'm sure I'm missing many, but that's all I can recall atm.

Life of Black Tiger, of course, with Mario Kart 8, Breath of the Wild, Splatoon and Xenoblade 2 as distant seconds.

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)

TETRIS THE GRAND MASTER 3 for the sport
Breath of the Wild for the relaxation.

Dragon Age: Inquisition

There is this one game that I have played since its release in 2015. Sounds like a good candidate. It certainly defined my whole gaming.

Bayonetta
Splatoon
Borderlands
DeadSpace

Horizon Zero Dawn. It blew me away like no other game that i've played in the past 10 years.

Mario Odyssey. That was the most fun I've had playing a video game in a long time.

Honorable mentions include Skyward Sword, BotW, Deus Ex Human Revolution, and Smash Bros Ultimate (it's only been out for a week lol). If we're still counting 2008, then i'd include Smash Bros Brawl and Dead Space.