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>
It's always a toss up between Xenoblade and Etrian Odyssey, but I've actually completed all the Xenoblade games compared to not having finished every Etrian game. I put Xenoblade X and Xenoblade 2 in my top ten games but I'm not sure I'd put any one Etrian game in my top ten, but I do think all of them are extremely good in general.
paper mario for the nintendo 64 and gamecube
as for general series with consistent output that i like across the board, i want to say portal but that has 2 games. uh, i like fire emblem and dmc, i guess