Now, as for the worst move ever. Bret Hart definately suffered as a result of his move to WCW, but I'm giving it to a man I've already mentioned and give it to Vader.
Vader was a multiple time Heavyweight Champion in Japan and held the same belt 3 times in WCW, taking the belt from Sting twice and Ron Simmons in his 1st African American Champion run, and found himself in Hulk Hogan and Ric Flair's main event scene during their runs as Champ.
So then he moves to WWF, and is basically portrayed as a thug. Following that, he feuded with Yokozuma and assaulted the big man, but yet had his WM match changed to a 6-man tag. A brutal feud against Ken Shamrock followed, and his WWE career was ended feuding with Goldust before jobbing to Bradshaw and Edge. You know it's bad once you reach that stage!
You'd think following this reduction in status, he'd find it hard to return to his status. Instead a return to Japan led to more gold in a short space of time. So... you made it big, so naturally you return to America and disappear in TNA before coming back to the lofty heights, of teaming with Goldust and Jonathon Coachman. Wow.
Vader... WWF/E really was never a smart place to go