Most WrestleMania appearances: The Undertaker - 26

The Undertaker has appeared at all but two WrestleManias since WrestleMania VII in 1991. He only missed 1994's WrestleMania X and WrestleMania 2000 due to injuries. That's a remarkable total. The rumors swirling about his lack of appearance at WrestleMania 37 this April are so notable because he's always been there.
The next closest total is Triple H, who has appeared in 22 WrestleMania events, starting with WrestleMania XII. The only one he's missed since then is WrestleMania 23, where he was injured.
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Triple H's total isn't that far behind the Undertaker's, but he reportedly had no match this year, and he'll be turning 50 in July. To break the Undertaker's record, he would need to appear five more times on The Grandest Stage Of Them All, and that's assuming the Deadman is actually done. He has a way of surprising (and frustrating) people every year.
The Undertaker is a personification of what Wrestlemanaia stands for and represents the grandest stage of them all in its truest sense.