The smart money says Cena. Apart from a brief feud when Cena was a heel, they've never really worked together in a long programme. Whether the more traditional fans like him or not, Cena is the biggest star in wrestling today and as soon as he won his first WWE title, he moved to Raw so they've nearly always been apart. They wrestled on Raw once since but that's about it.
If it does turn out to be Cena, and he is scripted to defeat the Undertaker, I'd consider that an insult as a wrestling fan. McMahon has come up with some truly insulting angles in the years since the WWE gained a monopoly on the "entertainment business" otherwise known as professional wrestling. That being said, it wouldn't surprise me if that's what the plan is for WM 26. Cena is only marginally better than Hogan as a wrestler, but nowhere near as good with the mic (which, these days in pro wrestling, is EVERYTHING). For him to be the guy that finally takes down the Undertaker? *sigh* I've never been a fan of Shawn Michaels.... at certain times, it could have been said that I hated the guy.... but at least he has a shitload of talent and would have been the most logical choice the WWE currently has to put a halt to "The Streak." Cena's a flavor-of-the-month whose fifteen minutes should've been up four years ago.