Since I watched all the Wrestlemanias, and ranked every single match not that long ago (about a year ago, I think), I'll go through and give my thoughts on who I think are the main contenders.
Bret Hart
The matches:
Battle royal (WM2, ranked #200) - Bret made it to the end of this, but there isn't really anything worth noting in his performance.
Hart Foundation/Danny Davis Vs British Bulldogs/Tito Santana (WM3, #73) - this is reasonably fun, but it's mostly due to the story of Davis getting his ass kicked. Nothing Bret did stood out, and it was actually Tito who I thought looked the best in this.
Battle royal (WM4, #205) - well, the only good part of this was the interaction between Bret and Bad News Brown at the end, but this isn't really any mark in Bret's favour.
Hart Foundation Vs Rhythm and Blues (WM5, #145) - this is forgettable, and the only thing I noted was that I liked the Anvil's hot tag work.
Hart Foundation Vs Bolsheviks (WM6, #237) - this is a seconds-long squash. Bret looked cool counting along with the referee for the pinfall though.
Hart Foundation Vs Nasty Boys (WM7, #51) - here we go, this is a good match. I will note that it isn't as good as the Rockers' match from the same show though, which is a point in Shawn's favour. Bret is good as the fip.
Bret Hart Vs Roddy Piper (WM8, #5) - Bret's first great WM match, and a really great storytelling match. This is my match of the night for WM8.
Bret Hart Vs Yokozuna (WM9, #54) - I thought Bret and Yoko always had good chemistry, and I don't remember seeing a bad match between them. I really liked that Bret would utilise a different moveset when he was in the ring with big men, using moves like the second rope bulldog and running dropkick. His offence looks good here, and there's a really good spot where Bret ties Yoko's foot in the ropes in order to take him off his feet. I think this is a better match than Shawn/Tatanka from the same show.
Bret Hart Vs Owen Hart (WM10, #2) - another great match. Bret's selling is really good here, and this match has a bit of everything. This is really a showcase for Owen, and Bret is really giving to get Owen's push going (as you'd hope he would be). My match of the night, though I don't blame anyone for preferring the ladder match.
Bret Hart Vs Yokozuna (WM10, #61) - Bret comes back to work another decent match with Yoko. A bit different to the previous year's match, as both men are selling their efforts from earlier in the night. I like how Bret just tries to go through Yoko here, as if he doesn't really have the energy to use his speed. A go down swinging mentality.
Bret Hart Vs Bob Backlund (WM11, #158) - this sucks. Actually, Piper ruins this with his comically bad ref job, but as a feud-ender, this really had no heat and no sense of hatred between the participants.
Bret Hart Vs Shawn Michaels (WM12, #9) - people call this boring, but I actually think they do a good job of peppering in the big spots to keep things interesting. I like that they both get to bust out some different offence too.
Bret Hart Vs Steve Austin (WM13, #1) - one of the best WWF matches ever, and this is really just a great example of two top-class workers who had terrific chemistry.
Bret Hart Vs Vince McMahon (WM26, #194) - really poorly laid out, which is odd for a Bret match. This really exposed Bret, which is unfortunate.
The case for Bret: four top 10 matches, including the top 2, and four MOTNs. Backed up by 4 other matches in the 'good' range.
The case against Bret: failed to make an impact at any of his earlier appearances (thought he didn't have the best opportunities), and has that one big failure with Backlund (hard to hold the Vince match against him).
Shawn Michaels
The matches:
The Rockers Vs The Twin Towers (WM5, #56) - this is a really good speed vs power type match, with both teams looking good.
The Rockers Vs The Orient Express (WM6, #179) - this is odd, because it seems like these two teams have no chemistry at all, when we know they had very good matches against one another on other occasions. Not even the best tag match on the show.
The Rockers Vs Barbarian/Haku (WM7, #24) - this is great, however, and I have this as the best straight tag match in WM history. All four guys looked good in this one.
Shawn Michaels Vs El Matador (WM8, #103) - this is okay, but I like Tito in it more than I like Shawn. I think it took Shawn about a year to really figure out how to be a good heel.
Shawn Michaels Vs Tatanka (WM9, #119) - another okay bout, but this probably went too long for a Tatanka match. There are a few bits of awkward-looking stuff, but this was a decent Shawn performance.
Shawn Michaels Vs Razor Ramon (WM10, #11) - Shawn's first great WM match, and this is a great performance from Shawn in particular, as he takes the big bumps and performs the big spots needed to make the match/gimmick work.
Shawn Michaels Vs Diesel (WM11, #66) - this is another very good Shawn display, as he brings big bumps and big offence like the splash to the floor to make his buddy look good.
Shawn Michaels Vs Bret Hart (WM12, #9) - see above.
Shawn Michaels Vs Steve Austin (WM14, #41) - Shawn really does his damnedest to put Austin over despite obviously being in a lot of pain. It's to his credit that I probably wouldn't notice if I didn't know.
Shawn Michaels Vs Chris Jericho (WM19, #44) - I think this is a good match, but that that is almost entirely down to Jericho, which I'll talk about more when doing the Jericho run-through. Shawn is a detriment if anything with his horrible no-selling of Jericho's back work.
Shawn Michaels Vs Triple H Vs Chris Benoit (WM20, #34) - Shawn shouldn't have been in this match, and he actively drags it down whenever he's in the ring with his crappy offence, crappy selling, and just general crappiness.
Shawn Michaels Vs Kurt Angle (WM21, #35) - yeah, I thought this was good, which goes against pretty much all my instincts. I actually prefer Angle in this, as he just comes across way tougher and more believable, and Shawn's selling gets a little over-dramatic.
Shawn Michaels Vs Vince McMahon (WM22, #190) - god, this is obnoxious as hell, with Shawn fighting off half the roster before destroying Vince. Even the big spot with the ladder takes an age to set up, and isn't that impressive. Vince/Hogan was way more fun than this, not to mention Vince/Shane.
Shawn Michaels Vs John Cena (WM23, #165) - I know some people like this a lot, but to me it's the two biggest perpetrators of the "action movie montage" selling that I detest (by which I mean it's all big, overly-dramatic slow-mo emoting) trying to outdo one another. There are aspects of this I liked, like how they built up to Cena's first punch, and the piledriver on the steps. I also thought Shawn's leg work looked pretty brutal (not that it meant anything, of course). It wasn't enough though.
Shawn Michaels Vs Ric Flair (WM24, #36) - some of Shawn's execution is pretty bad here, but he's clearly working his ass off to give Flair the send-off he deserves, which I respect. The match tells a good story, with Flair going into his bag of tricks to be the Nature Boy one last time, and Shawn being conflicted about ending his hero's career (the spot where Shawn pulls up from delivering Sweet Chin Music, only for Flair to lock on the figure four is really good). "I'm sorry. I love you" is just ugh.
Shawn Michaels Vs The Undertaker (WM25, #60) - I think this is a very good match, but not a great one. There's lot of drama, but digging deeper, the work doesn't really stand out as being all that great. Shawn goes a bit OTT with the selling again, whereas Taker's is generally strong. Good finish.
Shawn Michaels Vs The Undertaker (WM26, #7) - now this is more like it. Shawn reins in his more annoying tendencies, and the match is much more focused than the trading big moves-style of the 25 match. The finish is all-time great too.
The case for Shawn: lots of good matches, including 2 in the top 10, and, I think, 3 MOTNs. Maybe more good matches than anyone else at Mania.
The case against Shawn: some really poor individual performances, which I don't know that the WMGOAT should have.
Randy Savage
The matches:
Randy Savage Vs George Steele (WM2, #228) - Savage basically has to work around Steele here, and he manages to create a few entertaining spots, but that's about it.
Randy Savage Vs Ricky Steamboat (WM3, #13) - this is obviously really good, and I can't fault the work, but there are bits of it that I don't like, like Steele interfering, and the fact that Savage basically has the match won a couple of times before the finish. Nitpicky things I guess, but trying to explain why it's so low compared to the general opinion of the match.
Randy Savage Vs Butch Reed (WM4, #149) - nothing really to this, and it's short.
Randy Savage Vs Greg Valentine (WM4, #128) - these are running together for me to be honest. This was pretty solid though, but again short.
Randy Savage Vs One Man Gang (WM4, #242) - this pretty much sucks. Really these matches are just part of a bigger story, in which Savage has to overcome the odds in order to become champion. As standalone matches, they aren't really worth anything.
Randy Savage Vs Ted Dibiase (WM4, #52) - easily the best match of the night, even though the story isn't brought to a totally satisfying conclusion (with Hogan's involvement taking some shine off Savage's achievement). A good night's work for Savage.
Randy Savage Vs Hulk Hogan (WM5, #10) - a great heel performance from Savage. Everytime he gets ahead in the match, it's through skullduggery, like going after Hogan's cut eye, or choking Hogan behind the ref's back. He is really at the heart of everything that works in this match.
Randy Savage/Sensational Sherri Vs Dusty Rhodes/Sapphire (WM6, #173) - he's at the heart of everything good about this match, too, but not even he can save this from sucking.
Randy Savage Vs Ultimate Warrior (WM7, #3) - really amazing match and Savage performance (and Warrior is good, too).
Randy Savage Vs Ric Flair (WM8, #6) - Savage's selling is really good in this, even though his offence is a little lukewarm.
Randy Savage Vs Crush (WM10, #98) - this is short and sweet, with a creative finish.
The case for Savage: really stood out on those early shows, with the MOTN at 3,4,5,7 and nearly at 8 too.
The case against Savage: a shorter WM career than most of the other contenders.
Steve Austin
The matches:
Steve Austin Vs Savio Vega (WM12, #68) - this is a good start for Austin, as he looks really good in the heat segment, and the brawling is good here.
Steve Austin Vs Bret Hart (WM13, #1) - see above.
Steve Austin Vs Shawn Michaels (WM14, #41) - see above.
Steve Austin Vs The Rock (WM15, #94) - this is really a booking success, as a lot of the actual work between the two isn't great, with some clumsy brawling standing out. The weakest of their trilogy.
Steve Austin Vs The Rock (WM17, #4) - this is everything a Wrestlemania main event should be. Incredibly intense, and both guys perform really well.
Steve Austin Vs Scott Hall (WM18, #99) - I enjoyed this. They work out a really smart match, to make it as good as it could possibly be, including using Nash to keep things interesting. This is a big plus for Austin for me, because this could easily have been crap.
Steve Austin Vs The Rock (WM19, #8) - another fantastic match from these two, and the character work in particular is really great. The perfect end to their trilogy.
The case for Austin: he doesn't have a bad match or performance on his resume, which I don't think any of the other contenders can say.
The case against Austin: only 7 matches, with 3 of them being against the same opponent.
Chris Jericho
The matches:
Chris Jericho Vs Chris Benoit Vs Kurt Angle (WM16, #57) - this seems a bit high in retrospect, as I can't really think of too much I liked about this. In fact, I think it was rather forgettable altogether.
Chris Jericho Vs William Regal (WM17, #77) - I imagine these two had better matches in this time period. This was okay, sound psychology, and I liked the way the armwork came into play with Jericho unable to hook the Walls in. It seemed a bit rehearsed at times though.
Chris Jericho Vs Triple H (WM18, #86) - remembered as the match that went on after Rock/Hogan, but this actually features a pretty good heel Jericho performance, even though Hunter is totally boring as the babyface. Jericho's attack on the leg is nice.
Chris Jericho Vs Shawn Michaels (WM19, #44) - everything good about this match comes from Jericho's end, and I think it is one of the better performances in Mania history. His execution, his sustained targeting of Shawn's back, his mannerisms and mocking of Shawn- all top-notch work. This is a carry-job in my estimation.
Chris Jericho Vs Christian (WM20, #14) - Jericho finally gets an opponent up to the task, and the two of them put on one of the most underrated matches in Mania history, with a memorable finish.
Money in the Bank ladder match (WM21, #17) - the first remains the best, but Jericho is really just a bit-player.
Money in the Bank ladder match (WM24, #22) - the second best one, but again, Jericho is in the background. Probably should give him some credit from his involvement in the two best ones though.
Chris Jericho Vs Roddy Piper/Jimmy Snuka/Ricky Steamboat (WM25, #82) - this is all about Steamer, but Jericho first has to carry Piper and Snuka to something resembling a match. It's a good display.
Chris Jericho Vs Edge (WM26, #72) - this is a pretty paint-by-numbers match, which doesn't get going at all until Jericho starts working the leg.
Chris Jericho Vs CM Punk (WM28) - I did this ranking prior to WM28, but for what it's worth, I thought this was the best match on the show.
The case for Jericho: some really good individual performances, and usually found a way to make what he was involved in interesting.
The case against Jericho: a lack of truly high end matches.
Just my opinions, of course. I'd probably lean towards Bret, but then, I'm a massive Bret mark