Interesting topic.
Its a hard one though so I'll go through all the main events.
The main event in my opinion is the match that went on last.
So
WM1 - Hogan/Mr T VS Piper/Orndorff with Orton jr - I would say everyone of them deserved their spots. Mr T did his job and didn't hog much of the spotlight from Hogan but I feel that non wrestlers have no business being in the ring at all..so its a tough call really. But I would say T did deserve to be there.
WM2 - Hogan vs Bundy - Who did Bundy ever beat? I don't feel he was a main event guy.
WM3 - Hogan vs Andre - Both easily deserved their spots.
WM4 - Savage vs Debiase - Top talents who put on a good match.
WM5 - Savage vs Hogan - in my opinion Hogans best match.
WM6 - Hogan vs Warrior - Yes both deserved their spots.
WM7 - Hogan vs Slaughter - This match sucked!!! And who did Hogan beat to deserve the spot? Should have been Warrior in a rematch.
WM8 - Hogan vs Justice - Niether deserve this spot. Savage vs Flair should have gone on last. It was the WWF title match after all.
WM9 - Hart vs Yokozuna - Yes both deserved their spot here
WM10 - Hart vs Yokozuna - ....Owen Hart vs Lex Luger should have been it. Owen beat Bret that night and Lex beat Yoko as well! jk yes they both deserved it.
WM11 - Bam Bam vs LT - Neither deserves this spot. Made Bigelow look pathetic and I could never take him seriously again.
WM12 - Hart vs Michaels - Yes both deserve it.
WM13 - Sid vs Undertaker - Yes...because I am a huge taker mark and the right guy won lol. But yes they both did deserve the spot.
WM14 - Michaels vs Austin - Well...technically Michaels didn't deserve it as he didn't beat Bret...but yes they both deserved to be there.
WM15 - Rock vs Austin - Yes the top heel vs the top face.
WM16 - or WM2000 Trip vs Big Show vs Foley vs Rock - No the McMahon's did not deserve to be there at all. But I cant begrudge Foley his farewell match. Show needed the push really.
WM17 - Austin vs Rock - Yes they deserved it.
WM18 - Jericho vs Trip - Yes they did.
WM19 - Lesner vs Angle - Yes, great Main event.
WM20 - Trip vs Michaels vs Benoit - Trip shouldn't have been there. He was killing Raws ratings and no one wanted to see him with another world title. Michaels (c) vs Benoit would have been epic.
WM21 - Trip vs Batista - Trip was over pushing himself but at least did the job here. I think this should have been the end of Trips in ring career.
WM22 - Trip vs Cena - No way Trip deserved this spot.
WM23 - Cena vs Michaels - Yes both deserved it.
WM24 - Edge vs Undertaker - Yes both deserved it.
WM25 - Trip vs Orton - Trip should never have been in this match and definitely should not have won it! WWE needed fresh faces and Trip's ego was killing them.
WM26 - Michaels vs Taker - I feel the world title should go on last. This was a special match though and yes they both deserved it.
WM27 - Miz vs Cena - I didnt watch WWE during this time (or the 8 years before it lol) and what I did see of the Miz...well he sucks. But I can see WWE wanting a new face to freshen things up so will give him a buy. Yes they deserved it.
WM28 - Cena vs Rock - Neither deserved this spot. Should have been mid card. Punk was the main event that year.
WM29 - Cena vs Rock - Neither deserved it again. Rock should have never been champion. Should have been Punk vs Bryan. But they wasted that on the Australia tour.
WM30 - Orton vs Batista vs Bryan - Even Batista admits he shouldn't have been there.
WM31 - Lesner vs Reigns - I dont know...I dont watch WWE.
So out of all of them I would say...The Rock in WM 28. BS match which achieved nothing.