Well, my initial reaction has to go with Lawrence Taylor at WMXI but just to throw another name in the hat, Hulk Hogan at WMIX.
Bret Hart v Yokozuna for the WWF title, I wouldn't necessarily say Yoko was the most deserving guy on the planet, but for the era I think he was appropriate and they were building him up after winning the Rumble that year. So he successfully defeats Hart for the title after cheating, at which point a completely undeserving Hulk Hogan emerges to challenge Yoko "for Bret's pride" and after having no involvement in the story line whatsoever up to that point (he'd previously wrestled earlier in the night alongside Brutus Beefcake to take on Money Inc.) he enters the ring and quickly makes little of Yokozuna to end the PPV as the new WHC.
Never was an example of backstage politicking more obvious. He further showed how much he didn't deserve this honor by walking out of WWE later that year for a lucrative deal with WCW.
Actually it was the WWF at that time still and not WWE, as far as Hulk Hogan's winning of the WWF World Title goes, no matter how big a star Hulkster was, something tells me that Vince McMahon was the one that more likely than not made that call. Let's be honest, I loved Bret Hart's in ring work to death and he's easily a top ten of all time favorite. The performance against Roddy Piper for the IC Title the year before at Mania 8 was classic and of course Bret would only continue to outdo himself at WrestleManias afterwards, his double header at X (where he avenged his loss to Yokozuna anyway, which I think was really the plan all along even with them teasing Luger as the new Hogan) and his matches with Austin and HBK were the stuff of legend.
However, we're forgetting it's Vince's company at the end of the day and by saying that this was all backstage politics especially when I sincerely doubt you were even in Las Vegas that night, makes me seriously shake my head. Yes, Hogan had a long hiatus and came back and won the WWF World Title, there were some plot holes there, but at the same time, it is Hulk Hogan we're talking about, look at it this way, Randy Savage was almost out of action for a whole year and after only three months of being back in action he beat Ric Flair for the World Title. Is that really any different? I think not.
As far as Yokozuna being made short work of...well he already did wrestle an entire match against Bret Hart and Mr. Fuji's mistimed cheating did Yoko in. So it's not exactly a signature Hogan moment here where he soundly beat an opponent. Plus you are forgetting that a couple months later, Hogan dropped the title back to Yokozuna and put him over even further as a massive heel.
All my love to the Hitman here, but think about it, Yokozuna's victory over Hulk Hogan the biggest face of that time was a great way to go. Flair's title loss should probably have been held off and timed with an earlier Hogan return so Hogan could have won it back and then Hogan could have dropped it to Yokozuna that way. And therefore it probably would have made more sense to hold off on Bret's first ever title win until the following Mania since I think that part of the dynamic with him losing it and then Hogan winning it sucked, but just the same it still happened and we have to accept it. Also, Yokozuna had a HUGELY successful run as a heel champion too, not since Superstar Billy Graham did anyone hold the (W)WWF World Title as a heel for an extended period of time.
And in the end, Bret got his redemption against Yokozuna at the Mania after. Now I know there have always been these stories about Bret (even according to him) that there was supposed to be a Hogan-Hart match at SSlam 93, however I think a lot of what Bret said in regards to that is a work. Just like so much of what you see in wrestling is a work, even when it's not on a wrestling show. These people's jobs involve working a gimmick even when you don't think they are. So if you want to buy into all that backstage rumor crap as being gospel, be my guest, but seriously you really have to think to yourself why you'd want to believe an account of something that you weren't even a witness to. Seriously, man.
In fact if anything, Hogan did business the way it's always been done in wrestling, you go and you lose the World Title before leaving the organization to retire or jumping ship. To me it just seems like you are another anti-Hogan type that just wants to find yet another reason (an ungrounded one at that) to air your discontent.
But anyway I digress, Hogan was your pick and that's all well and good, but definitely for the wrong reasons, it's one thing if you didn't like the creative aspect of the angle, but to read you assume that you know why it REALLY happened is pathetic.
Now for my actual pick of least deserving Mania headliner, as much as I hate to say it because I think he was someone with great potential but it was Brock Lesnar, as unappealing in hindsight that Lawrence Taylor's one time Mania appearance was, we at least knew that it was the "First Time, Only Time, Last Time" as WWF billed it. However with Lesnar this was the guy that everyone thought would be the standard bearer. Such was not the case.
He goes and wins the biggest match of his career against a great competitor like Kurt Angle and a year later he decides he does not want to do this for a living anymore.
Say all you want about Hulk Hogan but the first nine WrestleManias built around Hogan all had a pay off and the portrayer of the character did the business the way it was supposed to be done. No one on this board can convince me otherwise and I would love to hear any and all rebuttals on my pick with Brock Lesnar, but I do think the record shows how that was a folly of WWE.