Some good ones:

Razor Ruddock
http://www.youtube.com/watch?v=rNo8nXD3xoc

Evander Holyfield
http://www.youtube.com/watch?v=rxDB4p2XoD4

Rocky Marciano
http://www.youtube.com/watch?v=gQEL5sd2aOo

George Foreman
http://www.youtube.com/watch?v=l2XlGWhY_5I

Roy Jones Jr.
http://www.youtube.com/watch?v=V1Khbw82qUM