Booker T
The “Book Man” came to Smackdown in 2004 after a Hall of Fame-level career in WCW and recovering from the god-awful Invasion. Immediately he made an impact by separating himself from the locker room, heeling up against John Cena in a great Best-of-Five series. He spent the next two years trying to legitimize the U.S Championship, while periodically challenging main event talent.
In 2006 he became King of the Ring and established a new persona: King Booker. Whether you liked the gimmick or not (personally I found it more annoying that anything else), you can’t argue with his success as World Heavyweight Champion, and finally becoming a main event talent once again. And hey, it only took six years to get back to that level.