Undertaker, probably. He's been here since the waning days of Hulkamania and has feuded and fought pretty much every big (and some not so big) name from each era - Hulk Hogan, Randy Savage, Jake Roberts, Jimmy Snuka, Vader, Bret Hart, Diesel, Shawn Michaels, Steve Austin, The Rock, Triple H, Mick Foley/Mankind, Kane, Big Show, Ric Flair, Brock Lesnar, John Cena, Batista, Edge, Chris Jericho, Randy Orton, CM Punk, etc.
This is why he and Cena would be an epic deal for Wrestlemania. Even putting the immortal streak aside, they can promote a match between Cena and Undertaker as the biggest opponent Cena has never defeated (Hogan and Austin don't count, they're retired), a guy who has literally battled with every major competitor throughout each age in WWF/E history. And then there's the fact that Cena is the guy has battled and defeated every major competitor throughout his own era EXCEPT Undertaker. Of course, when you add in the streak, it becomes that much more of a spectacle. (And yeah, Cena and Taker battled a couple of times with both guys scoring one victory each in 2003, but that was when Cena was a mid carder, it would be ignored in the build for this match.)
