#4 Diamond Dallas Page
One of the only WCW main-eventers to be signed to the WWF during the Invasion angle, Diamond Dallas Page could have been a World Title contender. He was a genuinely likeable guy with a good look, so what could've possibly gone wrong?
Everything! Everything went wrong.
DDP debuted in 2001 as the stalker of Undertaker's real-life wife (at the time), Sara. This would've been fine had they booked DDP as a credible threat, but they didn't. Instead, they had Undertaker completely thrash Page at that year's King of the Ring PPV. Despite this, DDP continued stalking Taker's wife and Taker continued to make DDP look foolish week after week. At some time during this rigmarole, DDP won the WWF Tag Team Championships with former WCW tag partner Chris Kanyon, only to lose them ten days later to the Brothers of Destruction at SummerSlam 2001. Also, he got pinned by Undertaker's wife... who isn't even a wrestler.
Drink that in for a moment - a three-time WCW World Heavyweight Champion got pinned by a woman that had never wrestled before.
From there, DDP became an annoying, Tony Robbins-type motivational speaker character, ditched the gimmick and went back to being a fan favourite, won the European Championship (which he held for about a month and a half), and left the company in April of 2002, less than a year after his debut.
If they just let DDP be himself, perhaps he would've been more successful. Instead, he was saddled with terrible storylines and gimmicks (he won Wrestling Observer Newsletter's "Worst Gimmick" award in 2001 for his "motivational speaker" shtick) and made to look like a joke against WWE-created main-event talent.
His WWE career wasn't a good thing - it was a bad thing.