To clarify, I'm not asking who was the most accomplished or successful athlete from another sport to transition to the sport. I'm asking who was the most athletic fighter to ever fight in mma. Athleticism is a multi-faceted concept that encompasses many different qualities including speed (hand speed, foot speed), power, balance, agility, reaction time, strength, explosiveness and many others. So, it's up to you to decide which of these traits you value more than others. As a preliminary criteria, let's say that you have to had an extended career in the sport to qualify. So guys like Ray Mercer or Alexander Karelin wouldn't qualify. Who are some fighters that belong in this discussion?
