Both are in their primes.
Rules allow headbutts only. No other form of attack is tolerated.
Who takes it?
Takedowns are a "form of attack" and disallowed. Holyfield.
Based on the title I thought this was going to be a stupid thread, but it's actually brilliant, lol.
Coleman had some nasty headbutts, but Holyfield is the grand master of the art form. Nobody does it like him. Coleman's head would be in pieces within minutes.