Aside from Mario, it's a tie between any other cute platformer (kirby, rayman, sonic, etc), and Pokemon.
I'm one of those "the newer gens are always better" people who feels Pokemon improves in every creative writing element every time. It really hit home with me on XY & Sun/Moon, I like that they're taking socially progressive routes in their world & character building.
Undertale comes at a close second. I love the story & characters, and don't get me wrong I do appreciate the dedication of Toby in making it by himself, but I really wouuuuuuuuld love better graphics for it. I hear rumors he's making a sequel, something tells me he's well funded now and could hire better artists if he wanted to >w>