#1 Vader
I won't lie - Vader is the reason why I wrote this list.
There was a lot of buzz surrounding Vader's WWF debut in 1996, as he was one of the most revered and respected pro-wrestlers in the world at the time. Having been a multiple-time World Champion in both WCW and New Japan Pro Wrestling, Vader wowed audiences all over the world with his combination of size, power, and agility. Surely, he was being groomed for a World Championship run in the WWF as well.
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At first, Vader was actually presented quite well, putting on a decent showing in the 1996 Royal Rumble and then attacking kayfabe-president of the WWF, Gorilla Monsoon, the next night on RAW. When he returned from his resulting "suspension", (which was actually a way to get him off TV so he could have shoulder surgery) he was placed in lackluster mid-card feuds with the likes of Yokozuna and Razor Ramon. Neither of these feuds were able to establish Vader as the machine he was for other promotions, as he won most of the matches in unclean or otherwise unimpressive fashions. But it seemed okay, because he was heading into that year's SummerSlam to compete for Shawn Michaels' WWF Championship.
And then he lost.
According to various sources, Michaels (who was a complete pain backstage at this time) was wary of putting over Vader because he was put off by his stiff style. He would spend the rest of his time as a glorified jobber to the stars. After losing a Mask vs. Mask match to Kane at Over the Edge 1998, he told Michael Cole in a post-match interview that "maybe 'Vader Time' is over", before calling himself a "big, fat piece of s**t". He left the company in October of that year, just over two-and-a-half years into his run.
I genuinely don't know how WWF managed to MESS this up so badly. By all accounts, Vader should have been WWF Champion. Instead, he won zero championships there. He should have been built like an unstoppable monster, much like guys like Brock Lesnar are booked today. Sadly, they had no idea what they had in Vader at the time, so he was built as "just another wrestler".
Absolutely heartbreaking