Yeah who knows how far ibeabuchi could have gone. As it is Sanders and Coetzee were probably the best of the bunch.
Accomplishment wise it's not even close. Sanders or Coetzee easy. H2H is where it gets tricky... As most of these guys were extremely vulnerable but dangerous operators.
I was going to say Azumah Nelson, but then I saw "heavyweight". I'll go with Ibeabuchi. Seems a fairly obvious choice, unless I'm missing something.