Tonga Fifita AKA Haku is definitely the strongest, here's some disturbing information about his badass-ness :
"Fifita made a name for himself outside the ring as one of the toughest wrestlers of his era and is known for allegedly biting off noses, and knocking people out with one punch, slap, or headbutt. Though he had an easygoing, friendly personality outside of wrestling, he had very little tolerance for people who tested him. Even renowned hardman Bad News Allen said in interviews that Fifita was without a doubt the toughest. His one time manager, Bobby "The Brain" Heenan said in a shoot interview that Haku was "the toughest man I've ever met in my life",[citation needed] but noted that he was also a good hearted family man. In 1987, Haku had a backstage fight with Jesse Barr (who wrestled at the WWF as Jimmy Jack Funk) which allegedly resulted in Fifita gouging one of Barr's eyeballs out, causing Barr to later wear a glass eye. After this fight, Fifita had a reputation that steered wrestlers away from backstage confrontations."
Lesnar is definitely in there too, so is Taker who actually f*cked up Chris Masters once in a house show I believe for refusing to sell for him, and also Jericho who apparently kicked Goldberg's ass in a backstage fight.