Father Time. Outside of that, I'd have to go with Joshua, although he is probably too green at this very moment.
Alexander Povetkin probably has the best chance of all, but only with a strong referee who stops KlitschKO from fouling like in the first fight - it was awful to watch. Any mentions of AJ will be immediately laughed at, has anyone even dared to throw a punch at him?
Thing about Wlad is he knows how to win. If he was in a fight vs a Parker or Joshua he'd just grind out a points win like he did vs Povetkin, Haye and Ibragimov. You can say he'll do it by holding and leaning on guys but that is still very difficult to do. Just ask David Price, maybe he should get some leaning and holding classes from Wlad.
There's being smart, using your height and strength and then there's outright fouling. I like Vladimir, but he fouled Povetkin all night long, and Pabon(?) did a shocking job that night. I really rate Povetkin, he looked great against Perez and will chin Wilder.