This is one of those "loaded" threads where there isn't ever really a right or wrong answer, only your opinion... many hate that Miz got to main event Mania, much less walk out the champion. But you absolutely cannot say he didn't deserve his chance because he was given it...Cena put him over in a no DQ match, Vince decided that and Cena had enough stroke to refuse it...so he DID deserve it in their eyes and the heat he got for it and "angry Miz girl" etc showed it was worth it. Was it who you WANTED to headline? No, but that's a lot different from deserving it or not.
Likewise, someone like LT, Floyd Mayweather or Mr T. were not wrestlers but bringing valuable mainstream exposure to the company, which translates to dollars. So they too deserve their placement as "Special Attractions".
So if you take YOUR ego out of the equation how do you decide?
The only way I can view it is who got to Main Event and either wasn't ready for it, or didn't really contribute much by doing so and there are two names that do spring to mind.
Yokozuna - The big man was by no means a bad worker and his title runs were not terrible, but he got that first one at Mania 9 FAR too quickly. They rushed into it with the first "Rumble Winner Goes To Mania" and it has ruined Mania booking ever since. How much of it was down to Flair leaving and Hogan's ego is up for debate. Had Flair not been on the outs then Bret might not have gotten the belt till Mania 9... Had Hogan not been so selfish with the awful ending, Bret could have been "injured" by Yoko after beating him forcing a vacated title to fill the first KOTR PPV rather than the clusterfuck that first reign for Yoko became. None of it is really Yoko's fault and his 2nd reign was given proper time and he grew into the role, so his loss at X to Bret was worth it...just. But Yoko for me is the one who wasn't needed with who they had at that time, better relations with Flair, better planning and checking Hogan's ego all could have prevented a lot of the issues that stemmed from what was the beginning of a near fatal slump.
Sid - Sid wasn't ready at Mania 8 and he was almost less ready by the time he got the honor at Mania 13 of walking in as the champion. Some of the same issues that apply with Yoko apply to Sid the year prior, the booking was skewed in Hogan's favour despite him not being "on the team 100%", there being better matches available and the whole angle and push being rushed. Yoko at least had the legit size/weight/agility combination to stand out as unique. You could see how Yoko became champ so quick but in Sid's case he was thrust into the final match of Mania 8 against a "retiring Hogan" after barely 8 months on the roster. The fans hadn't gotten used to Sid Justice as a face, much less a contrived turn used to cover a change in already announced booking. It was a slap to Flair not getting Hogan to put him over, but not a terminal blow. Savage was in many ways MORE of a dream match than Hogan was, but putting it 4th on the night was a slight Ric never got over, his attitude changed, he was dispirited and it led to his departure not even a year later. Just swapping those two matches round and NOT letting Hogan end the show would have probably meant Flair stayed around for a good while longer and Hogan would have gone to WCW anyway maybe just a bit sooner. They didn't even have to change the finish, Warrior still could have come out and Sid could have feuded with Savage or Davey Boy Smith for the IC title... Mania 9 would have been built around Bret v Flair and Sid could then have legit been ready to headline the following year.
Some others come to mind purely cos of the "multiple main event concept" but for the sake of clarity I'm gonna stick to closing the show... For some like Lashley and Umaga, they got to headline a Mania, their match was just earlier on the show but neither could be said to be legit show closers. Bam Bam gets unfair stick, he was facing the celeb, doing the job and getting WWE more mainstream interest for that year's show than either Shawn OR Diesel were whoever Pammy showed up with so he absolutely deserved to close the show. I'm not going with all this Mr. Perfect stuff, he never got over to that level. He COULD have had he not been injured and some other poor decisions were different (which I will get to) but the reality is he was never over enough, once he returned he was always playing the game of "one eye on the ring, one on the insurance" as he was claiming on his Lloyds policies, hence all his comings and goings. I'm not going to say Punk either... he could have closed the show, but Vince didn't give him that honor for a reason, we may not know what that is...but the last year kinda bears out Vince was right not to let it happen.
So my possibles for who should have:-
Rick Rude - No question he deserved to go to Mania 7 as the champion even if he did then job to Hogan. Vince was again, more worried about keeping Hogan and Warrior sweet at the time than actually developing the WWF talent pool to not need him. That it all blew up in his face within a year is almost poetic. Rude was the best heel in the business at that time and he deserved a good run with the WWF title, starting with a win over Warrior at the Rumble. Slaughter was Hogan's call, no one elses...he deemed Rude too small to job face him and Warrior was emboldened enough by that to refuse to job to him again too. The Iraq angle was poor, it didn't sell Mania well and it began the downward trend. That is on Hogan and Warrior and Vince for listening to them... Personally I would have had Mania 10/Bret two match thing happen at 7 with Hogan facing Slaughter without the belt. You could still have Warrior go over Savage in a career match, with the prize being the title match with Rude. Double duty for Rude could also have been done but perhaps in a Tag Team scenario... World Champ Rude and IC Champ Perfect teaming against Bossman and The British Bulldog would have been a great "opener". If they had let Rude have that, then he would have been around in 92,93 to work with guys like Lex, Bret, Perfect, Davey Boy and Shawn... the biggest issue WWE had at the time was they hadn't properly built anyone and had to rely on rush pushes like Lex and Diesel... A Rude who had been champ and headlined 7, even if he lost could have rebounded and faced Bret for the title 2 years later or Shawn 3 years later... he was definitely good enough, he made the US title more important than both the WWF and WCW World titles while he had it with a non roided body... something that could not have been lost on Vince as he was now without both Warrior AND Hogan anyway.
Davey Boy Smith - Again with Mania 9... this one is a little more difficult as in reality Davey was released cos of the roid trial. But make no mistake had he not been then Mania 9 could have been, should have been and very likely would have been headlined by the Bret v Davey rematch for Bret's title or even title for title and of course you could then set up the rubber match to give Davey the big belt finally... be that a Mania in the UK or in Canada the match would have sold huge.
Likewise in 1995, they were so intent on rushing Shawn v Diesel that they missed the chance to have Davey either in that match as a 3rd man or have him face Diesel and let Shawn build more before that blow off, perhaps as the ref. They didn't and went with a lacklustre Shawn v Diesel feud that REALLY didn't work. I personally think the first Triple Threat main event should have been there at WM11 and could have closed the show even over LT and Bam Bam. Davey might not have won it but the dynamic of two opponents for Diesel to face of different styles would have been a great way to build him as the champ, something facing Shawn alone didn't do.