#2 Daniel Cormier (22-3)

As of Saturday Jon Jones' greatest rival, Daniel Cormier, is retired. So this one is a bit of a stretch. He said he was done and most people agree that is the correct decision for him at this stage of his career.
However, given Cormier's farewell to the sport was somewhat tainted by an eye poke that left him with a torn cornea and unable to see out of his eye, it doesn't seem unlikely that at least a small part of him would be tempted to re-write the final chapter in his career.
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Perhaps renewing MMA's greatest rivalry could be tempting enough to pull Cormier out of retirement as he could attempt to finally get one over his arch-nemesis in Jon Jones.
At Heavyweight, one could argue, Cormier stands a better chance, as he wouldn't have to go through the brutal weight cut down to 205lbs. Jon Jones would undoubtedly like to get one over his rival once again and the UFC would stand to make huge money off the back of the fight. Maybe, if enough money is waved at DC, he could be convinced too. While this one is probably the least likely to take place, one can't help but wonder what if it will actually happen.