With all the talk of Best/Worst WM, I think the really important question is which WM actually had the longest lasting impact on WWE and to that I think WM21 is, by far (w/o really thinking it over) the most important WM, there has ever been or at least in the modern era.
To wit, 1) let's get the obvious out of the way, and note it create two main event stars. Other WM are lucky if they can create 1 star that won't flop within the year. But Cena is essentially still the top name and Batista is a damn movie star. 2) It was the first WM to really establish the UT streak (prior to this, it had really been more of a trivia question than a true storyline). Again, this is something that, arguably became the most important aspect of WM and it started here, 3) It established the MiTB, which is probably the most interesting and innovative concept WWE has come up with in years.
All 3 of these were monumental events with ramifications that affected the WWE up to this very day. That's pretty impressive when you consider WWE has trouble remembering what happened last week let alone 10 years ago.
While other WMs might have been better all around events (Which is debatable considering 21 is a 4 hour event with 2 classic matches and very little in a way of crap) or done better financially (again, debatable), I believe that, for sheer impact and long lasting influence, WM21 was truly best for business.
Good points, and it was also the biggest buyrate and highest grossing show up until WM23.
However, I will throw down the trump card with WM14, which not only launched Steve Austin and Rock as big stars, but actually resurrected the Wrestlemania concept from the dead. 11, 12 and 13 had essentially been glorified monthly PPVs and in fact 13 was one of the least bought shows of the year, so 14 was especially miraculous in that it suddenly made Wrestlemania into a drawing card on its own and set the stage for all the stadium shows and "Wrestlemania Play Button/No Number" stuff that followed.