In many ways, I haven't really liked this decade. Gaming-wise, there haven't really been many decade-defining games for me this decade. That's actually only two games. The rest on my will definitely stay in my mind but there's really only two decade-defining games, and even they pale in comparison to Metal Gear Solid 2 and Final Fantasy X from the 2000s. Anyway, on to the list!
There's just so many wasted hours here almost daily from 2012 up until this day and until foreseeable future. osu! is probably more like a lifetime-defining game than a decade-defining game, whether I like it or not. I was going through a fairly difficult period in my life when I got into osu!, and it's a great game too.
Perhaps Minecraft isn't all that innovative, but it was definitely the game that popularized whatever it is that it's doing. I've played Minecraft much less than I would have liked, and I have certainly played it quite a bit. Playing Minecraft isn't the only way to consume it though; There's also YouTube, which I've almost certainly spent much more time on (in the context of Minecraft) than playing the game. Like osu!, Minecraft really stands out from the rest.
3. The Elder Scrolls V: Skyrim
The problem with Skyrim is that it's even that great a game. However, it's impossible to downplay how big it is and how prevalent it is in the decade.
4. Metal Gear Solid V: The Phantom Pain
Had MGSV been just another game in the series, it probably wouldn't have ended up on my list - mainly due to clearly being unfinished. (It's still a great game even as it is though.) However, it's Kojima's last MGS game, which makes it stand out much more. It's the end of an era, which is why it deserves a place on my list.
5. Grim Dawn
Titan Quest was huge back in the day, and come to think of it, due to the amount of Titan Quest I've played this decade, perhaps Titan Quest should be the one on my list. However, my love for Titan Quest came to existence during the previous decade, so it wouldn't fit all that nicely on this list. Instead, I'll choose its spiritual successfor: Grim Dawn. Again, I've played Grim Dawn much less than I would have liked, but due to Titan Quest's legacy, it's quite special for me.
I had two strong contenders in addition to these five: Rocket League and Cities: Skylines. Rocket League is currently my most played game on Steam, and Cities: Skylines is also among my most played games. I also would have liked to have them on my list, but I strongly suspect that time will pass by them and they won't stay in my mind the way the five games I've listed have. Then again, only the top 2 games from my list really defined the decade for me, so including even more games would simply have been wrong.