It's actually not an easy one...
The First has Mokuton, is able to defeat Time/Space users (Madara, obviously) and can take Tailed Beasts under his control.
The Second was a genius in Suiton, no doubt possible. He was able to use Time/Space jutsus (though I think it's still unclear which ones) and he invented Edo Tensei (I think this is the name, I forgot tbh).
The Third was okay-ish, but doesn't have any special skills bar The Dead Demon Seal. Then again, he was good enough to battle First + Second, and he wasn't in his prime time if we're being honest.
The Fourth, Space/Time user, Rasengan inventor, Master in Fuuin. And he was a tactical mastermind, considering how he figured out Madara after such a short amount of time fighting him.
The Fifth.... Wel to be honest, just no.
My vote would go to the First, considering his abilities and the fact that he can defeat Time/Space users, without having that skill to himself, and he can hold up against a MS, considering he fought Madara while he had the MS.