I think honestly, it would/should be determined by the opponents they've faced. Edge fought a lot of good Wrestlemania matches that could've easily went against him. The T.L.C. matches count as matches that could've went either way. The MITB match counts as a solid match that could've went to someone else. His match against Booker T. at WM X-8, easily his match against Foley could've went against him.
Ultimately, I don't like Undertaker's streak being "impressive." He's defeated -- Jimmy Snuka, Jake Roberts, Giant Gonzales, King Kong Bundy, Diesel, Sid, Kane(2), Bossman, Triple H, Ric Flair, Big Show & A-Train, Randy Orton, Mark Henry, & Batista.
Jake Roberts basically jobbed big time to him, so did Snuka. Taker's character back then was SO dominate, that neither of those two stood a snowball's chance in hell. Taker v. King Kong Bundy was a joke, likewise with Taker v. Gonzales. Taker v. Bossman, Ric Flair & Mark Henry were all way too predictable. So boil it down, Taker has less than a handful of GOOD victories at Wrestlemania, against Triple H, Randy Orton, Batista, & Kane. As ANY of those matches could've went either way.