Answer ... the 6'9" herky-jerky boxer-puncher that rose from the dead to get a draw with Wilder last year ... Tyson Fury.
We talked to the 3x heavyweight champ who ruled the division for a decade -- who's calling the Errol Spence-Mikey Garcia mega fight on Saturday -- and he told us why he thinks his fellow countryman would cause him the biggest problems.
"I think Tyson Fury would've given me some trouble because he's so big. He's a bit lanky."
That's sayin' a lot ... Lewis was an absolute beast in the ring -- compiling a 41-2-1 record with 32 knockouts -- including wins over boxing legends like Mike Tyson and Evander Holyfield.
So, does Lennox think that Fury could actually beat him???
"I would break him down probably by the 10th round, I would break him down. But, he would give me some problems."