If he's retiring, then John Cena. Making this generation's biggest star your last victim is the perfect way for Undertaker to end the streak and close out his career, especially since he's never beaten a guy who was the #1 star of the company as far as Wrestlemania matches go. Shawn Michaels doesn't count since he wasn't the face of the company when he lost at 25 and 26 and was only top dog during one calendar year (1996.) John Cena has been the top dog for the longest that anyone has been (barring Sammartino) and has just about more kayfabe accomplishments than anyone in the modern Mania era.
If he doesn't retire, then Brock Lesnar. WWE tried to get this match going at WM27 with the real life brief UFC confrontation between Lesnar and Undertaker (which was supposed to get WWE a fair bit of publicity to hype the match, which doesn't make sense since it wouldn't correlate with Undertaker's Deadman character) but Dana White was the one with the power to allow Lesnar to wrestle a match in WWE and he said no. Undertaker also did an interview before WM29 where he said he wants to fight Lesnar at Wrestlemania before he retires.
No one else is even considerable, certainly not someone like RVD. I've pondered them giving a big push to Bray Wyatt by letting him get the Wrestlemania match with Taker, but I don't know. He's obviously getting a big push right out the gate and with him tangling with Kane and being a dark sort of character (which makes him a formidable opponent for Undertaker), I could see it leading into a Wrestlemania encounter with The Phenom later on if they don't go with the more obvious choices of Cena or Lesnar. But you could be guaranteed that it would pretty much kill Undertaker's streak of good/great matches (arguably the best matches of the card) of the last several Wrestlemanias starting with 23.