He was the greatest if we consider his achievements relative to his time. Went 132-3 in his prime, beat champions and top contenders from the lightweight division up to the middleweight division (and nearly captured the light heavyweight title). Remained a top contender into his 40's.
No idea how he would do nowadays, it's almost a different sport. The gloves were smaller, boxers fought every weekend, fights went to 15 rounds, judges scored on an entirely different basis. Referees barely interacted with the fighters, boxers fought in the clinches, and used dirty tactics.
Floyd Mayweather's style wouldn't have won him any favours in those days. His fragile hands would've been a problem too. George Benton was pretty damn skilled but he barely ever had a decision going his way.