Who would be more formidable H2H in later eras; Sullivan or Jeffries?

He had a fantastic jab. He routinely held his own in the jab department against much taller, longer foes.

Ali rarely went to the body, Holmes had a lazy left hand after the jab, Roy Jones didn't employ the jab much. Each fighter's style and physical assets dictates their emphasis in the ring, dictates their strengths and weaknesses. The trick is turning each fight into one that plays by your strengths and not your opponents'. While focusing on your weaknesses may give you a longer run, it lowers your ceiling for accomplishment. Peak Tyson achieved what few ever achieved. He cleaned out the division, completely reset it.

 

I'd say Jeffries had the most transferable attributes, he'd do better.

 

And he forgot about it. In several fights even in his prime.

Nor sure why you're bringing up random fighters. Never said Tyson was the only champion who had weaknesses in their fundamentals.

Yes, Tyson burned brighter than most in his brief window. But he could've burned longer than he initially did had he worked on those small weaknesses in his fundamentals. That was my entire point. To deny Tyson wouldn't have been a better overall fighter with a better inside game, stamina, clinching, etc is foolhardy.

 

Impossible to say how either of them would do but an easy observation is that Jeffries had better physical tools and also took on and beat established world class fighters in his day.