#3 Alistair Overeem
Back in December of 2011, Alistair Overeem finished Brock Lesnar in the very first round with a kick to the body. It was Lesnar's first fight in over a year, as he had been battling Diverticulitis for the majority of that time. So, it's safe to say that, back then, Lesnar wasn't ready to be in the Octagon with a guy like Overeem.
Then, following Lesnar's UFC departure, Overeem called the WWE star out after one of his wins, which was odd because Overeem had already decisively defeated him.
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The loss to Overeem is surely one that Brock Lesnar wants to get back, and if he does come back and fight "The Reem," it's certainly a fight that he can win.