Hmm... I voted for Jiraiya, that's because I think he's a bit stronger than Orochimaru, or at least as strong as him. That's the situation now, but I think the strongest person in the future (and at the end of the anime) will be Uzumaki Naruto. Of course also the 4th was very strong, but I think Naruto could easily become stronger than the 4th because the 4th only sealed Kyuubi with his own powers... But Naruto can use his own powers + the power of the Kyuubi.