Former UFC heavyweight champ Bas Rutten was just hitting his stride when a rash of injuries forced him to retire in 1999 at the age of 34.
The ever-improving Rutten learned from his early blunders and used those lessons to finish up on a 22-fight unbeaten streak. In that span, "El Guapo" won 11 fights via submission and seven by knockout. He also avenged two of his four career losses by besting Frank Shamrock (twice) and nemesis Masakatsu Funaki.
But Rutten didn't garner the No. 2 spot on this countdown solely on account of his impressive career-ending winning streak. Rutten landed at No. 2 primarily because of his aptitude to constantly evolve.
Rutten defined growth in MMA by balancing his strike-heavy attack with a rock-solid grappling repertoire. Like Shamrock, "El Guapo" prospered with unpredictability. Because his opponents didn't know how to prepare for him, they couldn't beat him.
Not to suggest that Rutten could get the best of UFC heavyweight champ Cain Velasquez, or even UFC light heavyweight champ Jon Jones. But having phenomenal athleticism and the propensity to adapt would definitely make Rutten a viable contender at either weight.