1. B.J. Penn
Penn, a first-ballot UFC hall-of-famer who was champion in two different weight classes and finished second on Sherdog's list of 10 greatest lightweights ever, also finished at the top of this more ignominious list. I didn't include him myself, but the rationale is obvious. Penn had very good wrestling, outstanding takedown defense and superlative BJJ in his heyday, but he was most impressive and unique for developing excellent striking as well, particularly his sharp, technical, fast, and powerful boxing. It was a seemingly unbeatable combination in the 2000s, far ahead of his competitors. Despite this, Penn wouldn't always come into a fight in shape or focused, resulting in fighting below his optimal level. He shouldn't have had drawn with Caol Uno after having annihilated him in 11 seconds in their first fight. He should not have lost the majority decision against Jens Pulver n or lost to Matt Hughes in their rematch, a fight he was easily dominating before gassing badly in the third, the fruits of not having trained nearly as hard as he could have. Those are all examples of underachieving.
Here is my issue, though. After Penn lost to Hughes, he dropped down to 155 pounds and gained new focus, finally achieving his tremendous potential. He destroyed Pulver in their, demolished Joe Stevenson to win the vacant UFC lightweight crown and obliterated another former UFC lightweight champion, Sean Sherk, with strikes to prompt a doctor's. Far from being done, he then delivered hellacious beatings to Kenny Florian and Diego Sanchez, tapping Florian in the fourth and forcing a doctor to halt the action in the fifth against Sanchez. At that point, Penn lost his title to another all-time legend in Frankie Edgar, which was an enormous upset then, but in retrospect, was simply a younger, more evolved great fighter beating an older great. From then on, Penn's struggles were not due to lack of training or effort. Rather, they were due to the tremendous increase in skill in the lower weight classes, which caught up to and even surpassed Penn's abilities. So while Penn's early career was an example of underachieving and he would have been remembered even better had he gone 2-0 against Pulver and Uno and 3-0 against Hughes, I don't think much would have changed from 2010 onwards.