#3 Brock Lesnar
Brock Lesnar arrived on the main roster in 2002 as "The Next Big Thing" who was intended to become that just that, the next huge WWE Superstar. He won the King of the Ring tournament and the WWE Championship just a few months after his debut.
The Beast won the 2003 Royal Rumble match and defeated Kurt Angle at WrestleMania XIX before wrestling his last match at the following year's WrestleMania XX event where he lost to Goldberg. His departure from the company came as a big surprise to many people.
When the fans found out he was leaving, they showed their displeasure by booing him and Goldberg, who was also leaving. After a two year run as a huge star at a young age, his exit was shocking and unexpected, especially since WWE didn't have competition anymore at the time.
Lesnar left because he wanted to pursue a career in the NFL, and he couldn't keep up with WWE's tough travel schedule. The Beast went on to compete in New Japan Pro Wrestling and UFC before returning to WWE in 2012 on the fall-out episode of RAW after WrestleMania XXVIII.