We've never seen a high level fighter take that much damage from another champion and not get stopped for 5 full rounds. Nobody else could have lasted that long from Cain and took that many shots and still kept coming. I used to think Roy Nelson was the toughest fighter in the UFCs HW division but Junior set a whole new standard. 
Whether or not he is the "toughest fighter on the planet" remains to be seen.
Impressive nonetheless.
saku
Hmm. We should add up how many rounds of punishement fighter's have survived. JDS so far went five rounds. Roy has been beating in how many rounds in total? 9 or so (JDS, Mir, Werdum). Penn took beatings from GSP, Diaz, and Rory.
Silhouette - saku
Yeah good point, Saku has taken some violent and vicious beatings.
Not many heavyweights would have had the chin and the heart to survive that
Note on Saku: legendary toughness, but many of his most violent beatdowns resulted in stoppages.
JDS just wasn't going to let that happen.
Clarence Parents - Note on Saku: legendary toughness, but many of his most violent beatdowns resulted in stoppages.
JDS just wasn't going to let that happen.
saku has been literally broken in the ring and has kept fighting a few times even pulling the comeback, the arona fight cemented to me that he is the toughest sob to ever fight
honorable mention to Big Nog, and Frank Mir what he came back from was incredible much like Nog
Silhouette -Clarence Parents - Note on Saku: legendary toughness, but many of his most violent beatdowns resulted in stoppages.
JDS just wasn't going to let that happen.
saku has been literally broken in the ring and has kept fighting a few times even pulling the comeback, the arona fight cemented to me that he is the toughest sob to ever fight
honorable mention to Big Nog, and Frank Mir what he came back from was incredible much like Nog
That is definitely true.. Saku was a legit madman and I'd never question his toughness.
We've only seen JDS take one serious beating which makes it harder to judge where JDS really stands in the equation.
JDS vs Mole Man for the title of "toughest fighter IN the planet."
...UFC 158, under the MGM Grand Garden Arena.
What was amazing to me how dang TIRED he was after the first round.
then still took a beating and wouldn't quit.
I think he will come back and take his belt. He wants it more than Cain.
Can't wait to see the rematch!
He's tough, but he not the best anymore. It clear he doesn't have power after the first couple rounds. He has the heart to survive but he can't do any damage in the clinch or on the ground. 
Do you guys not remember Sonnen beating the hell out of Silva for 4.5 rounds?
Shogun or fedor
miltonm1977 -He's tough, but he not the best anymore. It clear he doesn't have power after the first couple rounds. He has the heart to survive but he can't do any damage in the clinch or on the ground.
Well we saw both Big Country and Carwin go the distance where JDS couldnt put them away but still dominated over the course and was never in trouble.
The only thing we know now is that Cain has been the only guy to exploit what you mentioned. 9 guys before Cain, incl Cain earlier, could not do so.
JamesTheWelshDR - Shogun or fedor
Yes