Undertaker would be the smart guess, but I honestly can't discount the Sting theory.
He has yet to re-sign to TNA, WM and the HoF are in Atlanta ("WCW Country"), there needs to be a "big" hook or two for WM this year, WWE is putting out a Best of Nitro dvd later this year, Nash and Booker are already there (Nash supposedly signed a Legends contract), Sting is already 51 and WWE would provide the perfect closure to his career, if there's ever a Best of Sting dvd, WWE has the footage and all to do it, Sting's been critical of WWE product in the past but it's more PG now, etc etc etc.
In all honestly, if Sting were to sign to WWE, it would be right now. And judging by some of these wrestling boards online, the general consensus is 1) grown ass men who would readily admit they would "mark the fuck out" if Sting-Taker happpened and 2) there would be a LOT of people not thinking of ordering WM, but would in a heartbeat if this match happens.