I can only go on what i have seen while alive...

Foreman demoliton of Smokin' Joe....it just wasn't possible, but there you have it!
Ali KO's Foreman...nobody expected that...
Duran V Leonard 1...the biggest fight outside the Heavyweight division
Hagler V Hearns....i mean wow!
Tyson obliterating Berbick announcing his arrival as invincible?