Multiplayer: League of Legends. I probably have spent over half a year or more of my life playing it over the past decade cumulatively (which is 1/20 hours of my life during that span). I got pretty good at it during my junior year of college and made it to my promos into Diamond in ranked. Now I mostly just use it to chill with my friends and talk while playing the specialties modes. It is my comfort food as I am so familiar to it.
Single Player: Celeste/Cuphead. I sat here trying to decide which one to choose but that's foolish. I'll just pick both. They both scratch a very similar itch for me and drove me to my limits in so many good ways. Celeste even got me to the point where I was perfecting/speedrunning levels. Cuphead is maybe the most beautiful game I've ever seen. Both feature godly soundtracks.
Honorable Mentions: Ori and the Blind Forest, Dead Cells, What Remains of Edith Finch, Wolfenstein: The New Order, Mario + Rabbids: Kingdom Battle, Virtue's Last Reward, Danganronpa v3, Uncharted 4, Persona 5, and Baba is You.
Games I Still Need To Try: WAY TO MANY