I think surely it must be Mairis Briedis. A guy who never cracked the ring top 10 p4p lists. Which looking back on is shameful. He is probably the 3rd best Cruiserweight of all time and gave a prime Usyk a harder fight than Fury and Joshua. But the man never got a shred of respect from anybody other than the most hardcore boxing fans. Artur Beterbiev is another good shout. Spent most of his career outside the top 10 p4p lists. Only now has barely cracked the top 5 which is long overdue. Has knocked out everybody he's ever fought. Never had much of a hype train behind him. But again, it must surely be Briedis. By a long shot IMO.
Usyk before he entered the HWs. Only I, CST, Serge and some other here knew he would wiped out all his opponents even in this category.
Ioka and Chocolatitio are probably in the running. Choc was briefly P4P no. 1 but not for long enough and hasn't received the praise he should for a true ATG and 3 of his 4 losses were to dodgy decisions. Ioka is a fellow 4-weight champion and the best counterpuncher in the game today yet seldom features on P4P lists despite having a very deep resume. Knockout CP Freshmart also should have featured more on P4P lists I believe. Chris John seems to have been mostly forgotten as well unfortunately.
Amnat Ruenroeng maybe? Pretty dirty fighter but I always felt he was underrated. He beat Ioka, Casimero, Shiming and Arroyo. Also gave Rungvisai a close fight despite being like 40 odd years old.