For me personally, any game in the Super Smash Bros series.
They've always been a go-to when hanging out with friends, in their time.
Melee still has a following. Nobody likes brawl because of tripping (among other things i'm sure), but I learned how to mod brawl and added so many custom stages and character skins. Sm4sh was ok, and Ultimate is so complete.
Yeah it's Nintendo and "it's always moral to pirate Nintendo games" and there's plenty to complain about their online services but I've played smash so much that every time a new game comes out immediately a must-buy.