I can smell the heated polemics over this subject from very far away.
Anyway, my experience with Mario RPGs is relatively limited, having only played SMRPG, Paper Mario, Super Paper Mario, Sticker Star, and Colour Splash at varied points in time. Of those that are fresh enough in my memory to be able to form some opinion on, I consider the original Paper Mario to be my personal winner; it offers a streamlined but fun turn-based combat experience, a good amount of exploration, and a well-paced, fun story in terms of what Mario games should be fit to tell. Sticker Star and Colour Splash collectively come second for putting further emphasis on exploration, and even despite the shortcomings in their battle systems, I honestly think the consumable attacks offer a layer of strategising that was frankly not present in more traditional entries and thus can only make the experience more unique. I do still prefer the traditional approach, though.