#7 Último Dragón
By the time Último Dragón signed to WWE in 2003, he was a sixteen-year veteran of the business with multiple championships under his belt. Most of his success came in his native Japan, but he was also a rather decorated luchador in Mexico. Needless to say, there was quite a bit of hype surrounding Dragón's debut with the company.
And then, they did nothing with him.
Despite an impressive victory over Shannon Moore in his WWE debut and despite having an awesome/marketable look, Dragón was almost immediately put on the back-burner. The reason for this was that they expected Dragón to be a high-flyer like Rey Mysterio. When they found out he was more of a hybrid competitor, he was relegated to C-shows such as Velocity and Heat, until we finally got to see him go up against Mysterio on SmackDown. Dragón won, but only because of Tajiri's interference. The match was wholly forgettable, too short, and ended with shenanigans, which is a shame because they could've made this a major PPV match if they had built up Dragón more effectively.
Following his match with Rey Mysterio, Dragón had one more match on SmackDown before disappearing until WrestleMania XX, in which he competed in the Cruiserweight Open. Though he scored the first elimination of the match (eliminating Shannon Moore), he was forced to submit by Jamie Noble soon after. He'd wrestle a couple more times with the company before asking to be released in April of 2004. His request was granted, and he went home to Japan.
For his brief WWE run, Último Dragón earned Wrestling Observer Newsletter's award for "Most Underrated" in 2003. Given his less-than-impressive career there, it's pretty easy to see why.