Who in your opinion is the greatest creator/developer to have worked at Nintendo, past or present, and why?

Yokoi. He pretty much created Nintendo, and it is his blue ocean philosophy that guides the company to this day.

Honorable mention: Eiji Aonuma. Developer responsible for Link’s Awakening, Majora’s Mask, and even mainline 3D Zelda since (IIRC?). Spotless track record!… unlike Miyamoto, who is infamous for sending projects into shambles with his “flipping the tea table” philosophy, which while often for the better of a project, it can sometimes lead to games like Sticker Star.

Going to second Yokoi for the same reason as Firebush03. His philosophy guides Nintendo to this day.

I know you said one, but sometimes it's a pair. Specifically the Miyamoto-Tezuka pair. Miyamoto is the more public facing figure of the two, but the two of them designed-directed and created multiple of Nintendo's most iconic franchises, and headed up some of their best games.

Since the late Wii era, they've mostly split into separate categories of games with Tezuka handling the 2D stuff and Miyamoto the 3D... both with exceptions, they both work on Pikmin and both worked on Zelda Breath of the Wild.

Shigeru Miyamoto

Mario, Zelda and Donkey Kong are just incredible franchises. He has also pretty much had a hand in every great Nintendo franchise ever released.

Obviously Miyamoto but i want to mention Koizumi and Itoi for trying to push serious storytelling in Nintendo games (Link's Awakening, Majora's Mask / Mother).

Reminder that Yokoi was very opposed to the idea of the Gameboy as we know it but it was rather thanks to Okada's team that it became more than a glorified game and watch.

This statement needs some additional context. Yes, Yokoi wanted the most affordable design for GameBoy, but he ultimately did support Okada’s proposal. This matter boils down to nothing more than a difference in vision and should not tarnish his overall contribution which arguably made the GameBoy the success that it was.

That said: What was his contribution? Well, one if his major contributions was in making the system significantly less powerful and, in particular, being without color output. In doing such, Yokoi intended for the system (a.) to have a solid battery life and (b.) be sold at an affordable price. If it weren’t for Yokoi’s intense advocation on this matter, the system would’ve likely been in a situation very similar to the GearGear, a portable system which did output color at the cost of being over priced and with a very short battery life.

Matter of fact, had it not been for Yokoi scrapping together the GameBoy production fiasco after a fatal production error caused GB to temporarily be canned, there would have been no GameBoy. So, again, I don’t believe a single difference in vision should discredit Yokoi’s overwhelming contribution to the GameBoy’s massive success. (Need I remind you that this individual was solely responsible for setting up GameFreak with small projects to pump up funds, in order that they may produce a little know franchise called Pokémon?)

No Miyamoto, no Nintendo. Any other answer is incorrect... with perhaps Yamauchi as the lone exception.

Sakurai, even if I think he's bad at picking DLC fighters.

Hardware wise it's Yokoi while software wise it's Miyamoto.

Nintendo chose the roster for the second fighter pass, but admittedly the rest were on Sakurai.

I suppose I have to pick Shigeru Miyamoto. I do think he has outstayed his welcome, but we wouldn't have so many iconic Nintendo IPs without him. Mario, Donkey Kong, The Legend of Zelda, etc. He even had some involvement with IPs he didn't create like Animal Crossing. 

Honorable mentions to Gunpei Yokoi, Takashi Tezuka, Eiji Aonuma, and Yoshiaki Koizumi.