It all depends on who comes in shape and who doesn't. Dexter won last year not so much because his muscularity and presence dominated onstage in the way Dorian or Ronnie did when they were at their prime, but more so because he had the least flaws while having a bit of shape and the best conditioning. Nearly all of the fans of bodybuilding still want a big guy to win the Mr Olympia and in the context of bodybuilding, Dexter is not a big guy.
Dex has been beaten by a lot of the other competitors before, so if any of them show up in better shape than he is in, it's theirs to win - I'm talking mainly Jay Cutler and Victor Martinez. But Victor has had a knee injury and his legs don't seem to be anywhere near what they used to be when he was in form (like the 2006 Mr Olympia).
Guys who could beat Dexter if they come in top shape include Jay, Victor, Phil, Wolf and even Toney. And even though I hope it never happens, I think Kai Greene could beat Dex too, given he's more freaky and bigger. I personally would like to see Victor or Wolf improve enough to win it - Dex just doesn't suit being Mr Olympia due to his underwhelming presence onstage.