Surely Kimbo Slice is miles ahead of any other fighter ever in this respect.
On the back of his youtube fame he was one of (if not the) most famous mma fighters out there when he took up the sport.
He was quickly headlining events and companies were staking their very existence on his fame and future success.
He never even began to live up to his hype and suffered a pathetic first round loss to a UFC reject before entering The Ultimate Fighter and losing his first match there (and refusing an option return to the competition)
Despite his complete lack of success he was still given a shot in the UFC on the back of his notoriety and although he actually managed a win he was soon losing and being cut.
Brock probably the next most hyped fighter, although to be fair to him he did become the Champion so it's a little unfair to say he was over hyped (but that just shows the level of hype he was given that even a title win and defenses were way below par for built up expectation)