I always thought Hearns would be the toughest match up for Floyd out of the Four Kings. His power, his reach, his ability to box, it would be hard for Floyd to outbox Tommy, I think he'd have to fight his ass off and get inside after taking some punishment and do what SRL did. He would have to try to fight Tommy in the pocket or inside, that would be an uphill battle for Floyd, imo.
I think Julian had the ability to always land a big shot and he had atg power. When we talk about p4p biggest punchers, Jackson is almost always mentioned first.. So yeah he could knock out just about anyone in his size range, including Floyd. That being said I don't think his skill set is anywhere close to Floyld's and Floyd would be cautious early and find a way to adjust like he always did. Jackson was a huge puncher but not very more than one dimensional overall.