In no particular order,

Ray Mercer (notice how we can say his last name alone, and we all know who he is even though he wasn't an elite champion of the era)

George Foreman (the batterings he took against Stewart and Holyfield in the second half of his career would have killed an elephant!)

Jack Dempsey

Ali (Ali carried too many of his fights when he could have very well ended them. Thus, he took more bruisings than most champs.)

Rocky Marciano (although not that high on my list because he fought a lot of over-the-hill guys and really never been in that many wars EXCEPT for against Jersey Joe.)

Tony TNT Tucker (that's right, and fuck y'all who don't think so too. He stood up against Tyson in his prime and Lewis at the latter part of his prime and took what he had coming to him- and finished all of those fights. You can't talk about tough SOB's without talking about Tony TNT Tucker)

Honourable mention: Frans Botha. (that's right, and fuck y'all with this one too if you don't think so. There is NO WAY Botha should have been in the ring fighting for championships and taking the champs at the time to the limit- he was beating the snot out of Moorer and Tyson before he got ko'd. Botha made it that far on pure grit.... )