Schaub if he didn't lack a chin.
BJ (in hindsight) if he would have had a real coach/trainer.
Fedor (yes Fedor) for multiple reasons. One being his unwillingness to split from M-1. Another being his unwillingness to modernize his training methods and camp. The third one being his unwillingness to cut to 205.
Either Diaz brother. Both of them are extremely talented, but neither has shown a willingness to change their game plan, whether it be fight-to-fight or round-to-round no matter who they are fighting or how that fight is going.
Arlovski is another prime example of a guy who was/is unfathomably skilled but lacks a chin.