I'll start with James Buster Douglas who absolutely gave Mike Tyson a real beating in 1990 .
This was meant to be a warm up fight for Tyson as he prepared to take on Evander Holyfield later on that year .
Nobody gave Douglas a chance , they had to host the fight in Tokyo because nobody in America would have paid a cent to watch it .
This wasn't some plucky no hoper landing a lucky knock out punch , he actually beat Tyson up good and proper for 10 rounds before knocking him out .
Biggest upset in boxing bar none in my opinion .