Iwata, easily.
The DS, the Wii, and the Switch as his legacy - these are Nintendo's three most successful consoles of all time. And this direction can be directly linked to him while Wii U I attribute more to Shigeru Miyamoto's influence since the console did not resemble Iwata's descriptions of what Nintendo's next console should be, but mirrored Miyamoto's exactly. Although, the 3DS, which was also somewhat of a failure, fit things Iwata was interested in (like 3D).
Crediting Yamauchi with getting Nintendo into the video game industry is not really accurate: Gunpei Yokoi, a toy and game (table top games and such) inventor working for Nintendo, got the idea for video games and is the one who actually first moved Nintendo in that direction. Yokoi is also the one who put Nintendo's first development teams together. Speaking of Gunpei Yokoi, the Wii, the DS, and the Switch are all part of his legacy as well, since those products were all designed with his philosophy in mind for getting a perfect product for Nintendo's execution of the blue ocean strategy - which is kind of Iwata's input - in creating a product for a market segment that is undeserved either in large part or completely.
Yamauchi also did a lot more to hold Nintendo back in the N64 and Gamecube dark ages. Doing his very best to insult as many second and third parties as possible, and fans. Had he NOT been CEO of Nintendo, had it been Takeda or someone else, I feel the company would have been better off.
Iwata, on the other hand, had a great mind for the video game industry, he wasn't just some guy sitting in a chair signing corporate cheques, carousing with other wealthy Japanese business elite, and buying boats and sports teams. Iwata was actually a thinker, someone who intended to fix Nintendo's problems, and he did, even in the light of overwhelming competition, he found a way to win.
I describe myself as a little dose of toxic masculinity.
