It needs to be a big-name guy. The Undertaker has put over enough young guys in his career, he doesn't need to do so anymore, especially at Wrestlemania. When I see this question, only a few names come to mind.
John Cena: As much as I dislike Cena, he's the biggest name in pro wrestling. The Undertaker is at the point in his career where he should be wrestling guys with names as big as his. Taker can put on a good match with just about anyone, including Cena. This would be a HUGE face vs. face feud, and the title wouldn't need to be on the line. The only problem is, I'm not sure the WWE wants Cena to lose at Mania, especially in a one-on-one match. And I really, really don't want to see Taker lose at Mania.
HHH: Yes, it's been done before. I don't care. You can recycle feuds by creating interesting storylines. This would be another huge face vs. face match. These are probably the two most respected guys in the locker room, and with good reason. Either guy can, and has, put on a quality match with anyone you can think of. They make other people look good, including each other. I think this is the best "wrestling" match he could have. But I would like to see HHH be The Undertaker's last Wrestlemania opponent, and 27 might not be Taker's last.
Chris Jericho: This match has never taken place at Wrestlemania, and I think these two could have a dynamite heel vs. face type feud. Jericho becomes obsessed with beating certain people, and who better to target than the Undertaker. I have said before, and will say again, NO ONE should beat Taker at Mania. Taker would go over clean, but Jericho doesn't need a "rub" from anyone at this point in time.
Last year, I wanted to see an Orton vs. Taker Mania main event. But with Orton turning tweener/face, I no longer want to see it. Heel Orton vs. Face Taker would have had an AMAZING build, but with where they both are currently, I just don't think it would be as entertaining.