I voted for Smash Bros. in this thread, but if going strictly by a Mario series, I'd vote for Mario Kart, albeit a bit hesitatingly, just because I'm very burned out on that series and I don't wish to pick it back up any time soon.
RPGs all the way for me. I enjoy a lot of the other Mario spinoffs but none even come close to how much love and nostalgia I have for the first 3 Paper Mario games and all of the Mario and Luigi games (excluding Paper Jam).
As the previous comment says, this poll is missing the sports games (e.g. Mario Tennis, Mario Golf, and if crossovers are included, Mario & Sonic at the Olympic Games) however I personally wouldn't have voted for this anyway.
Idk to be honest. I think it's sort of a three-way-tie between Mario Kart, Mario Party, and Mario RPGs. They all are replayable and never get boring to me. And Mario Kart and Mario Party are great to play with friends in person or online.