Aside from the judicial system, the toughest opponent Jon Jones has ever faced is Alexander Gustaffson, who went five round absorbing and dishing out a ton of punishment. Jones won that fight, but eventually he lost the belt for being an idiot.
Then, it was up to Daniel Cormier and Gustaffson to determine who deserved to be king.
Cormier took a heck of a beating in that one, and came away the winner. Now Cormier and Jones will get it on at a UFC 197 (in a rematch – Jones won handily the first time).
My sparse words are probably doing little to convey just what sort of war was fought between Cormier and Gustaffson. Thankfully, the UFC has released a video of the fight. Watch and enjoy.