errrr since spoilers aren't allowed I'll go with the 3rd. Not in his condition now, but when he was in his prime. In his prime he was the greatest Ninja ever, but due to old age he isn't anymore.
I think the 4th was also very strong.
The thing that annoys me the most is that people are saying that the 4th is stronger than the 3rd because the third had to step aside from his position(not in this thread). I don't think that's the case. I think that the 3rd chose another hokage because of his old age. Going back to the story, the 3rd was Jiraiya's and oro's sensei when they were lil genins. Then Jiraiya became the sensei of the 4th when he was jounin. I bet that the 3rd was in his 50s or even 60s when he chose the forth. and now since Oro and Jiraiya r 50 years old the third has to be in his 70s or 80s. That's why he isn't the best anymore.