The greatest of all time was Blendo:
Designed by Jamie Hyneman (of Mythbusters fame). It consists of a 5hp lawnmower engine powering a circular saw blade, encased in a dome made from a wok. The thing was so dangerous Jamie almost lost a foot starting it up (using the ripcord to start the lawnmower engine).
It was undefeated. It tore the ever-loving hell out of anything it faced, frequently launching dismembered parts of its opponents into the crowd at terrifying velocity. It was simply too dangerous to be allowed to continue competing, so it was retired!