Marciano by my estimation. He didn't stop swinging until the guy was on the floor. Even when he had guys hurt bad, he would punch guys as they were falling. Tyson was relentless too but past 5 rounds he rarely stopped anyone.
johngarfield would be good to ask about this, his knowledge of boxing is phenomenal. I might PM him the link later today.
As far as who was the most relentless while exhibiting world class talent, with the best physical attributes like speed, and power. . . well it's gotta be. . . HAWKTIME!!! Aaron Pryor. . .
I agree with this, along with Henry Armstrong. Those guys were relentless but had explosive speed and power to go with it. Guys like Margarito and Lamotta were relentless but more plodding so not as dangerous imo.
Ricky Hatton IS up there. Not the most skilled, but strong, fast, vicious and you can be sure he's coming for you.