Kind of obvious, since Hulk Hogan is the biggest star, period. So naturally he's the biggest star at Wrestlemania. Besides, look at his archive of matches at the event. The biggest, most important match, for the business, that Shawn Michaels was ever a part of at a Wrestlemania was his loss to Steve Austin at Wrestlemania XIV because it was the coronation of the Austin/Attitude Era in 1998. Michaels "sort of" toppled Bret Hart as the face of the New Generation two years earlier, but his drawing power was never remarkable. For match quality, Michaels is difficult to beat, and would win this poll between these three quite easily, considering the moderate number of stinkers both Hogan and especially Undertaker have had at Wrestlemanias in the past.
Hogan was the driving engine behind The Rock and Wrestling Boom, embodied by his partnership with Mr. T at the first Wrestlemania. Two years later, well, yeah, Hogan vs. Andre. Two years later, the Mega-Powers Explode. Randy Savage was awesome but it was Hogan's star power that made Savage an icon because the angle made him a pseudo-equal (not really "equal," but the entire angle put him just underneath Hogan's level, which is nothing to be ashamed of), the next year, The Ultimate Challenge with Warrior, a "passing of the torch" that never really was, the next year, eh, the conquering of the Iraqi Turncoat (silly angle, but it did get very modest media attention), two years later he steals the show in the wrong way by taking Bret's title from Yokozuna, then comes back nine years later for one of the biggest Wrestlemania matches in years against The Rock, then caps if all off in a feud with Vince McMahon the following year.
Nobody even comes close to that sort of quintessential canon. Even the true heavyweights of Wrestlemanias past: Andre, Savage, Michaels, Undertaker, Austin, The Rock, Triple H, Cena, Batista. Nobody.