Basically, who was the consensus best fighter in the world for the longest? Uninterruptedly. Meaning that, during that "reign", they didn't momentarily lose that title then regain it.
Manny Pacquiao. Ranked as P4P no. 1 for four years on the bounce by The Ring from 2008 to 2012. Mayweather is second because he had two three-year long stretches of being ranked no. 1 between 2005 to 2008 and 2012 to 2015. Chavez and Sweet Pea both had three-year reigns one after the other. https://boxrec.com/wiki/index.php/The_Ring_Magazine's_Annual_Ratings:_Pound_For_Pound--2000s
Roy Jones Jr. was p4p from around 94/95 to when he lost to Tarver in the rematch in 2004. So about a decade. I can’t think of anyone off the top of my head who beats that.
Short- SRR Hard to compare guys who existed before the phrase and rating convention. P4P was a promotional tool to promote Robinson so I'd expect to see him with the most mentions and as Reinhardt points out his run isn't bad either. Be hard to make an argument against him, imo. But I would be curious, Nonpariel, Fitzs, Burns, they had their time in the sun and I think maybe if the mentality existed back then they might rival. I don't think the 20s is much beyond propaganda, most of it after the fact. Greb isn't even listed until recent revisionism. But just before then, when the power behind boxing were struggling with one another as well, maybe worth looking at closely.