UFC ref John McCarthy said the strongest person that he ever grappled was Mark Coleman (in 1997). John said that Mark was an absolute freak as far a brute power & force were concerned.
MMA outlawed all the techniques that Mark used to beat people with (headbutts & knee's to the head from side-mount and north/south. Eventually all he had to go to was his signature neck cranks.
If headbutts were legal, he wouldve crushed Pete Williams & Pedro Rizzo within 5 minutes. Those fights were '98 & '99.
The only dude to beat Mark (without Mark being supressed by rules) was Maurice Smith (props!)
Kerr and Coleman were very similar fighters, I think Kerr was the better wrestler.