It's hard to argue with either Russo or Vince McMahon as both were clearly ego trips... they "could make themselves" champion so they did. Arquette wasn't great but there WAS a marketing idea behind it at least, however damaging it actually was.
Vinny Mac had no need to win the title having won the Rumble, that in itself was enough of a "landmark" for the character. Him taking the title seemed sell-aggrandizing at its worst. Expecially when the logical move would have been to give Shane the title, as he actually could go in the ring enough to make a "shock win" stick.
Russo had literally no reason other than "he could so he did". He'd always wanted to be one of "the boys" and failed to grasp that by giving himself the title they all worked so hard for he was probably gonna ruin his credibility with all of them... he genuinely was that clueless.
But I'm not going with those two or any self booked reign, so that takes out Nash's win over Goldberg only to lose to Hogan too... it happened for decades where the booker got himself into the title position, so it's not really on to pick one over the other... all the two Vince's really did was what Nash, Duthty, Flair and others had done.
It's also easy to pick someone like Khali, who was horrid in the ring but offered a legit physical presence that made sense... it's fairer to say Rey was worse than Khali... at least Khali was bigger than most of the roster whereas major suspension of disbelief is needed for Rey to beat anyone at all.
So to me here are the contenders:-
Sgt Slaughter: As a worker he wasn't terrible, but the whole title angle was so misjudged and hurt the WWF rather than helped it. Sure the numbers for Mania were "good" but they had to move down from the outside venue planned, not for safety of Slaughter as they claimed, but cos successive bad champs in Warrior and Slaughter had killed the desire of fans to attend. It also cost them talent like Rick Rude, who felt slighted that the next stage for him was taken away and given to a Hogan crony, that had a knock on effect for several years. Rude winning instead of Slaughter means that guys like Bret, Perfect, Smith and the like all come along quicker...and thus more talent is "over" to main event level. Slaughter's reign was never about him, his abilities or even the Iraq war...it was about Hogan making a power play that backfired and led to him being gone... You can imagine Vince wanting someone like Rude or to go different and Hogan doing the "trust me brother...this will be huge..." it wasn't...
The Miz - This was the ultimate WTF? more so that Rey who at least had the storyline following Eddie's death. Miz is the same size as your dad, as annyoing as your little brother and as good in the ring as your mum. There was no reason for him to get the title other than the "conveyor system" WWE has used fruitlessly for nearly 30 years... if a talent DOESN'T get the World title within 2 years of their mid card title, they failed and get fed to the next one... it doesn't work as a system and Miz is the best example of how badly it doesn't. Talents that should be secure, valued and happy in the midcard get thrust into spots that are never gonna work, thus damaging them forever. It'd be like making Rick Martel WWF champion in 1990 and beating Hogan at Mania... it just wouldn't have worked. Miz has improved since, probably in the hopes of another go but hopefully he's worked out that the midcard is not some "desolate place" but a great place for a long, lucrative career... a title run/Mania match might have been good for one payday... but it's taken him YEARS to get back to being relevant and even now it's Sandow who is actually over, not him.
Shawn Michaels - Before you tear me down... I am talking pre-hiatus Shawn... we can all acknowledge that there were, in reality 2 versions of HBK... the one in 2002 was infinitely superior to the first. That first title reign for Shawn, indeed the first 2 were the most misguided thing WWF EVER did and it nearly cost them the company. Shawn was horrible in that face role, unrealistic as a champion considering the monsters he was facing and he screamed "It's FAKE" through his actions... this is the guy who only jobbed the belt to Sid..and Austin when Taker was in the wings in case he didn't... he didn't lose his lower card titles, always having to be suspended or hand it over... pre "retirement" Shawn was an awful champion... only the DX/Screwjob angle managed to get him any traction at all...and then he was hurt and gone. His later work, top notch... he learned... but those early runs, utter garbage and like Slaughter, cos them talents like Vader.
But even then he isn't the worst...
Again a controversial view but I am gonna go with:-
Dolph Ziggler - Another product of the conveyor, someone who had no business EVER being a World Champion but cos guys HAVE to be elevated or let go, he got 2 goes at it... His first was blink and you'd miss it... indeed I am amazed they refer to is as an actual reign. But his "big push" last year, which ended with the concussion was so misguided... Remember he got MITB by injuring someone else, so he was not the first choice... but the guy who was next. When he got his chance, while you can blame injury for ruining it it was HIS style that meant he was at risk of it.
Match wise I get why people rate him highly, and he IS improving, this push I think WILL stick... but Ziggler's "runs" have actually damaged his career, to the point now where you'd almost want them to "reset the clock" if they are seriously going to have him pushed this time...just like in some ways I and Shawn would probably take those early reigns off if they could... but all you can do is "not focus on them". Again it's cost them talent in some ways, as in the original plan Barrett would have been a champion by now, and a much better main eventer. Thwagger kind of fits this too, his reign while not as bad as people make out actually hurts his chances of a "proper one" more than his faux pas before Mania...even though he now could probably handle it, people don't remember the DUI but they remember the shit reign...
Basically ANYONE associated with that conveyor system is immediately gonna struggle unless they are ultra talented... those that get past it are the legends and HOF guys like Jericho, HHH, Rock, Austin, Angle, JBL, Eddie, Cena and Edge... the ones who struggle are the guys without that extra "It factor" who are just given it cos they are next in line... Christian, ADR, Dolph, Swagger, Jeff Hardy, Rey, Big Show, Miz, Orton and Batista (their earlier runs were also the drizzling shits) and even Shawn in his early incarnation.