Kane
picture courtesy of wwe.com
While I personally am not a fan of this option, at this point it would make more sense.
A re-masked Kane is something many fans have wanted to see and there is a contingent of fans who want to see vintage Kane and the Undertaker have one last match. The appeal can't be denied for anyone who has been a fan since the 90s, but it wouldn't do much for the current product.
Still, it is worth remembering that it would have the emotion of two men who have worked closely together and that would shine through in their performance.
The major thing holding it back is Kane's involvement with Cena and the fact that the Undertaker is more of a neutral character now.