#6 Tazz
Before Brock Lesnar owned Suplex City, there was Tazz, the "Human Suplex Machine".
He earned this moniker in ECW, where he regularly tossed opponents around like rag dolls. He also had an extremely-protected submission finisher, the Tazmission, which won him multiple championships in the promotion. Between his literal-and-figurative breakneck mat wrestling (see: Sabu) and his intense promos, Tazz seemed to be a legitimate badass.
A top AEW star just called one of his colleagues 'spoiled' and 'narcissistic' HERE.
And then he was signed to the WWF.
Granted, he did have a fairly impressive debut at the 2000 Royal Rumble when he became the first man to defeat Kurt Angle. However, his success was short-lived. While he was ECW Champion, he was booked to lose to then-WWF Champion Triple H, which was seen as a symbolic slap in the face from Vince McMahon to "lesser promotions".
He quickly moved down the card, turned heel, feuded with Jerry Lawler, and was quickly defeated by Stone Cold Steve Austin, who allegedly refused to put over a 'midget' like him (many wrestlers apparently shared this sentiment). He did end up winning the Tag Team Championship with Spike Dudley in 2002 but lost it to Billy & Chuck not long after. He would then be relegated to colour commentary for the remainder of his run.
To be fair, he did look kind of ridiculous standing across the ring from much bigger guys like Triple H. But still, for a guy who was practically built up like a precursor to Brock Lesnar, Tazz's WWE career was pretty laughable.