No doubt about it, Amano is a professional artist with his own unique style. Many of his illustrations have an ethereal, wispy feel. And I believe Amano definitely shows great mastery over the principles of his art.
Tetsuya Nomura came to prominence and completely changed how fans viewed the world of subsequent Final Fantasy games. Of course, Nomura’s sci-fi (and even goofy) illustrations won over Sakaguchi and Square, replacing Amano’s D&D inspired artwork. This leads many to cite Final Fantasy as losing its focus.
Say whatever you want about Nomura, I don’t think you’ll confuse too many of his designs with each other. Layout Nomura’s character designs. Tell a causal player to order them by game. I think that person will put most of them in their proper place.
Amano’s character designs lack diversity in contrast. Many of his characters could easily find themselves in other games (or many high fantasy settings). This is because Amano’s Final Fantasy worlds were heavily inspired by D&D, to the point where they borrowed (or plagiarised) elements from them. Plus, Amano often reproduced the same visual experience and created similar worlds each time.
Nomura, on the other hand, seems to create distinct world building in many of his titles. You won’t find the “Sculpture” enemy from Final Fantasy VII in the semi-realistic world of VIII, the post-apocalyptic world of X, or the semi-futuristic world of XIII.
Nomura’s enemies don’t make sense unto themselves. Still, many of them make sense within the context of their own world…usually. And each world has its own identity. One of Sakaguchi’s vision included no sequels. Nomura designed art with their own worlds. He helped make sure no one would think it was a sequel simply by a glance.
Ironically, however, Nomura’s titles ended up getting the most sequels…