Like Mir earlier on this list, The Prodigy has literally conquered some giants in his time. He moved up two weight classes to take on and defeat Rodrigo and Renzo Gracie in 2004 and 2005, respectively. Penn also went the distance with, though ultimately lost to, Lyoto Machida, who outweighed him in their fight by about 30 pounds.
Penn tops the list for his ability to take on and defeat larger opponents, but more so for his ability to chop down the largest trees in his own neck of the woods.
When they fought in 2004, welterweight champion Matt Hughes was seen as unbeatable. That didn't stop a relatively green and unproven Penn from choking Hughes out in the first round. He also pulverized Hughes in their rubber match just a few months ago.
And this is to say nothing of his long runs atop the UFC's lightweight division. Then again, B.J. typically has to venture outside his own weight class to find himself in an underdog situation. But that role seems to suit The Prodigy just fine.