BJ Penn could have been a world class boxer IMO, and was a BJJ world champion in his day--Nick Diaz is world class in boxing and BJJ as well. But you're asking the wrong question IMO, because seeing an art in action in it's pure form you get the benefit of ignoring the reality of a real fight-- Its the ability to transition between arts that produces the best MMA fighters anyway-- As the saying goes, it's not always the best fighter, it's who fights best.
It's why you don't see the best guys in individual arts necessarily excel at mma, their arts often fail to take into account the strength of other, different arts, and have strategies that make them succeptible--wrestlers have the best shot at adapting and becoming proficient at a second art of all outside arts, because they can generally control the transitions and often have spent their entire life learning how to learn an art.
A great striker is never so great with the looming threat of a double leg... A great pure boxer is often using tactics that don't take into account defending against grappling, for example a very sideways stance that makes them a more narrow target in boxing but also a sitting duck for leg kicks and single leg takedowns-- A great kickboxer will have to be much more careful with their kicks, particularly body and head kicks, for fear of ending up on their back. The muay thai straight up and down stance has no answer for wrestling, so it must be addapted in the same manner as a boxing stance for actual mma-- MMA easily exposes the holes in individual martial arts, thus it is in many ways a superior form of competition and proof of what Bruce Lee belived--take the best from each, discard what doesn't work.