I think you can make an argument for putting him at #1. He has legitimately good wins over Brandon Figueora, Angelo Leo, Daniel Roman, Arnold Khegai and Julio Ceja. Figueroa is a proven commodity with legitimate B tier wins vs Luis Nery and Mark Magsayo, and he was a reigning champion when Fulton beat him.
Some boxing and sports websites had Fulton in the top 15 P4P before he faced Inoue. I don't think Inoue's other opponents were ever ranked that high P4P, so Fulton is up there. It's too bad Gonzalez never fought Inoue when he was near the top of the P4P list. That would have been a nice name on Inoue's resume.
Donaire was frequently top 10 P4P for years (2009-2013), as high as 3rd at times: https://en.wikipedia.org/wiki/List_of_The_Ring_pound_for_pound_rankings I don't know about Inoue's other opponents but it's likely that Fulton was overrated P4P as he was certainly overrated (not to say he's bad at all but objectively he underperformed relative to expectations) going into this fight vs Inoue.