Yoshitaka Amano VS Tetsuya Nomura VS Akihiko Yoshida

[Wolf Kanno]

Basically what Fynn said, most sources I've read have said that Sephiroth's design was jointly created by Nomura and Amano, much like Setzer and Shadow from VI were a joint collaboration. It's pretty obvious too considering Sephiroth's Amano design is not actually a big departure from Nomura's while Cloud and Squall's designs have some serious variation to them.

If you want to argue impact, I would still defer to Amano, as Nomura has said in many interviews that he pretty much idolizes the guy and his work on Edea was largely Nomura's attempt at creating a character in a similar style to Amano's character designs. Considering how utterly faithful Nomura was in redrawing Amano's character designs in his own style for Dissidia shows the dude has nothing but respect for the master.

[Shiva95]

Tetsuya Nomura, but they're all great.