This is a very hard question. I could pick something like Tekken 8 to force myself to finally become half-competent at a fighting game for once, (Laugh). More realistically, I might pick something like Sid Meier's Civilization 6, even though it's hard to pick just one strategy game. I'd pick this one because it'd also be the most likely for me to play with other people to be honest.
I would find it absolutely impossible. I know myself. If I like the game I come back, but it is very difficult for me to keep playing only one game that long without alternating with other games. The closest I've come is playing "World of Warcraft" and "Neverwinter Nights online".