All of them together really, including Hiroshi Yamauchi. But for the most part probably Gunpei Yokoi and Miyamoto.

If Gunpei Yokoi didn't created the Gameboy (with the Video Link Cable), there would be no Pokémon, and Tajiri would be worthless.

So Yokoi-sama.

Easy there would be no pokemon without the gameboy

Gunpei Yokoi easily

I'd vote for Yokoi. But why not include Shigeru Miyamoto?

No love for Yamauchi?

Gunpei Yokoi set the basic rules that Nintendo still follow today and is the reason they did well when they first came into gaming he's probably the most importent figure they've ever had as Yamauchi tasked him to lead the company into console gaming. He created the handheld market with game and watch (design is very similar to the DS) before going on to revolutionize it with the gameboy, was in charge of the development of the NES, mentored Miyamoto, co-created Metroid and lead the basic direction of the company. Only man in Nintendo who comes close is Miyamoto who he trained when the legend joined Nintendo, Iwata in future may come close as well for turning Nintendo around.

Yokoi is the most important person in Nintendo's history. End of story.

What I was going to say... I honestly don't remember Pokemon saving the gameboy...

I mean... what was even challenging the gameboy?  PSP is the most successful non Nintendo handheld and... come on even without Pokemon PSP wouldn't be close.