Yeah, it's Jerome Le Banner and it's not even close, not even by a mile. The dude is probably the greatest King without a Crown. He has fought virtually everyone in K-1 and then some. He's also beat pretty much everyone (and lost to a few).
I think he would have definitely been the K-1 Champ in 01 had he not broken his arm. Sad.
I think the guy who would be after JLB would probably be Crocop, if you look at it, he has a VERY impressive career in K-1, beating almost every name.
Oh, and I'm gonna give a shoutout to Mike Bernardo. The guy has literally fought everyone that has stepped foot in Japan (albiet he has lost to everyone of them) The guy is a trooper and supplied many fighters with great performances.