In terms of current, I would have to agree with Valuev ..... can you imagine a guy getting a tap on his shoulder and hearing "is there a problem?", then turning around and seeing 7'2" of giant, misshapen, hairly Nikolai looking over him like some crazed Wookie.

All time, it's got to include (in no particular order)

Sonny Liston
Harry Greb
Mike Tyson
Marvin Hagler
James J Jeffries
Dwight Qawi
Jack Dempsey
George Foreman


But my all time favourite wild man and general uncontrollable hardcase:

Stanley Ketchell