AJ Styles and Batista are 5/2 joint-favourites.
In a post published last week, I mentioned that the Wrestling Observer Newsletter reported that Cena's match - as of the time of writing - was scheduled to be bigger than for the WWE Championship. This should rule out AJ Styles if the current titleholder is retaining all the way through WrestleMania season.
Dave Meltzer - in his article - stated that, when learning of the plans for Cena, that the names that stood out to him were Batista, Goldberg, The Undertaker and a long-shot in Hulk Hogan.
All four appear on this list. I've already noted Batista as a joint-favourite at 5/2, Goldberg is 11/4, The Undertaker is 7/2 and Hogan is 5s.