You can now play only one game for the rest of your life.

I hate to say it, but League of Legends. As much as I might love any other game, almost none of them would be re-playable for the rest of my life. At least with LoL, I can look forward to a patch every two weeks and a new champion/rework every now and then.

Probably Final Fantasy XIV. An MMO came to mind, and FFXIV is my favorite, so I'd be fine playing that. Plus there's social interaction and events.

Its a toss up between these 2;

Super Smash Bros Melee
and
Hello Kitty Cube Frenzy.

Melee is so fun with all the crazy fighting styles and levels but hk cube frenzy always has such soothing and cute pictures to look at after you beat each level.....ugh its so hard xD I love these two games both so much! Do I HAVE to choose? *puppy dog eyes*

I'd probably and regrettably settle on World of Warcraft (you may substitute any MMO for the same intentions) because you can choose what to do and make your own fun and deviate from the path the game wants you to go (AKA "zomg grind to max level and do raids all week"). It's hard to explain in a short summarized post why but in this case it's more about having a lot of options than it is about preferring the gameplay, since it's definitely not about that. I could name 20+ games easy I'd rather the gameplay in.

I would probably pick mario, since it's a platformer, it has even glitches and more things to do i guess.
No, it would be impossible, since i cannot focus in only one game at the time.
Or probably pok?mon, since the first time i have beaten red and blue, i can't think of getting bored easily.

GTA V. It comes with Online, so I'd probably be entertained for quite awhile.

I've played it 3 full times already and something always prevents me from finishing, yet it still never gets old per playthrough. And when you mix in Online, it's basically infinite.

For now, it?s Splatoon 2.