John Bradshaw Layfield. Remember when JBL was the longest-reigning WWE Champion in the history of SmackDown!? Yup, nobody else does, either.
Averted with The Great Khali. Although he was marginalized a bit following the loss of his World Heavyweight Championship to Batista late in 2007, WWE managed to keep him sufficiently menacing up until his Heel-Face Turn a year later....and thenthe decay set in.
Also averted with Umaga. Although he did lose three consecutive pay-per-view matches in early 2007 after having previously gone undefeated for nearly nine months, he managed to remain a dangerous (if hardly ever successful) villain right up until his death in 2009.
Played straight and then subverted with Montel Vontavious Porter. He was the longest-reigning United States Champion in the title's 33-year history until Matt Hardy finally won it from him in 2008. From there he began a long slide into irrelevancy, until by the fall of that year he was stuck in a months-long losing streak that made you forget he had ever been at all competent. But this eventually proved to be a blessing when the crowd began to cheer him out of sympathy, resulting in a Heel-Face Turn and a long-denied victory when he won a match that enabled Triple H to participate in the 2009 Royal Rumble. He then went on to be just as successful as before, even defeating Shelton Benjamin to become United States Champion once again.
Lampshaded with Edge. After winning the World Heavyweight Championship in December 2007 (which he had never lost, as it had been stripped from him earlier that year due to an injury), he decided to safeguard his title by forging a strategic relationship with SmackDown! General Manager Vickie Guerrero. Although at first he only pretended to fall in love with her, as time went on he genuinely became enamored with her and transformed into a slightly effeminate character who swooned over Vickie every chance he got. In any event, the strategy was an enormous success: Edge had Vickie set up numerous scenarios in which his chief rival, The Undertaker, couldn't possibly win, and persuaded her to have 'Taker banished from WWE entirely. Edge proposed marriage to Vickie in the summer of 2008, but the ceremony ended in disaster after it was revealed that he had been cheating on Vickie with Alicia Fox, the wedding planner. Vickie told Edge that she hated him, which led to a brief period of Cry for the Devil as Edge begged Vickie for forgiveness. But Vickie would have none of it, and decided to avenge herself further by having Undertaker reinstated to WWE and booking him against Edge in a Hell in a Cell Match at SummerSlam. This turned Edge into a wild-eyed coward who didn't believe he could win the match without help, and so he sought advice from Mick Foley on how to win a Hell in a Cell Match. Foley flat out told Edge that he was going to lose the match, because ever since he had fallen in love with Vickie he had been a complete sissy....
For the Divas, Molly Holly. This two-time Women's Champion was forced to undergo a protracted Humiliation Conga that lasted almost a year after WrestleMania XX, when Victoria defeated her in a match, strapped her to a barber's chair, and shaved off all her hair. Eventually her hair grew back, but things only got worse for Molly when Stacy Keibler(of all people!) pinned her in three consecutive matches. This was truly the beginning of the end for Holly, because if someone so inexperienced could pin a former champion three times, then who couldn't? It wasn't long before "Mighty Molly" wasn't so mighty anymore - indeed, was a complete joke, losing every single match more or less regularly before finally leaving WWE.
Her problems began back in 2002. When she did her Heel Face Turn, she revamped herself as a self-righteous prude and never got any heat because she was always easily silenced by the hot girls she ranted at by them just taking their tops off, leaving her with her jaw hanging open in shock. It didn't help that Molly wanted to play it as a comedy character, intentionally wearing the most matronly-looking outfits possible in situations where something sexy was called for. It got still worse when WWE unleashed the infamous "Molly's got a big butt" angle and was made into a complete joke, even though she was Women's Champion. WWE eventually realized they made a mistake and had her temporarily Take A Level In Badass in 2003.
Happened at an alarmingly fast rate with Tensai. He went from a true menace, defeating John Cena AND CM Punk (In a handicap match, but still...), two of the WWE's top stars, but within a matter of months, became nothing more than a big man for underdogs to prove themselves against.
Ryback, as a face, was a guy who never lost without a lot of interference and anyone only came out over him they had a big numbers advantage. Almost every match he was in was a complete Squash Match, and in pay-per-view main events, he was in control the majority of the time. When he turned heel after WrestleMania 29, his opponents generally got in a lot more offense against him, although he still won non-title matches. The real decay set in when Justin Gabriel injured his leg, which led to him having to quit a match against The Miz of all people the following week. At the same time, he ended up in a feud with Chris Jericho, which did nothing but make him look like a spineless coward, and indeed, only beats Jericho with a roll-up pin during the pay-off match. He also walked out on a match with Mark Henry. Rather than being a Wrestling Monster, he merely acts as The Bully to non-wrestlers backstage.