I think the following matches probably qualify, I know there can technically only be one "best", but these set the standard of what "Wrestlemania" should be.
Wrestlemania 1: Hulk Hogan & Mr T. Vs Paul Orndorff and Rowdy Roddy Piper. These was built well, booked well; and started hogan's build. Fact is: Hogan wouldn't be Hogan in the absence of Piper and would probably have never gotten to WM 3 and Andre.
Wrestlemania 3: Hulk Hogan Vs Andre, for the reasons all pointed out. A lot of people forget Andre's back was already going, and that he took this match to continue the business and to "give back" to it.
Wrestlemania IV: Not the best Wrestlemania, but Savage wrestling match after match and making each look like something special was definitely a raising of the bar of its own way, culminating in him becoming the champion.
Wrestlemania 5: Piper's pit with Morton Downey Jr. Not a "match", but definitely a "wrestlemania moment" that is remembered by anyone who saw it.
Wrestlemania 6: Warrior Vs Hogan. Memorable, especially considering it was supposed to be "the handing of the torch." This was back before we knew anything about the warrior other than the character portrayed.
Wrestlemania 7: Oddly, Virgil vs Ted Dibiase is probably the most memorable match, built up by Virgil being trained by Piper.
Wrestlemania 8: Piper Vs Bret Hart. Definitely a highly charged emotional match. There was the tearing friction of friends fighting over a title and giving it their all; and you waited on the edge of your seat to see who would end up winning. Piper had beaten The Mountie because Hart literally (as in not part of the intended storyline) was out with 103 degree fever.
IX: Hogan/Yokozuna/Hart - Not as big as it could have been, and not as memorable as other matches.
X: Owen Hart vs Bret Hart, again, Hart usually did the "emotional rollercoaster" matches, and was basically king of these types of matches. Luger/Hart/Yokozuna was also decent, but the Harts were more memorable.
11: Not as many good matches. Diesel (Kevin Nash) vs Michaels and Bret Hart vs the "snapped" bob backlund would be the standouts, but don't compare to earlier WM's
12: Iron Man Match - Hart Vs Michaels
13: Hart Vs Austin.
14: Austin Vs HBK with Tyson as special referee, again, not as good as other matches, but decent enough to be remembered.
15: Austin/Rock
16: Dudleys/Hardyz/Edge & Christian Ladder Match, as well as Benoit/Angle/Jericho
17: Angle/Benoit, Rock/Austin/, Dudleys, E&C, Hardyz
18: Wrestlemania 18, good matches, but none stand out as groundbreaking
19: Rock/Austin - Lesnar/Angle
20: Benoit, HBK, HHH
21: MITB
22: Cena/HHH, though again not as good as earlier matches
23: None really stand out.
24: Michaels vs Flair attempted to do the older version of Hart's matches. Decent, but not comparable.
25&26: Michaels Vs The Undertaker for both, these matches were reminders of what a wrestlemania match could be.
27: Undertaker Vs Triple H.