Were obviously speaking hypothetically, because no one would ever actually be able to defeat him. Kishi wouldn't allow it.
Anyway, the setup for these fights is really important. What I think we are talking about is a 1v1 fight, with no interruptions, where both fighters have the intent to kill. Special allowances can be given to fighters with special abilities that need preparation or other considerations. Kabutos Edo Tensei is the prime example. Think of it like giving them time to gather their weapons and equipment. Obviously some fighters are at a disadvantage if they are engaged in battle without their kunai, or katana, or scrolls.
All the people who actually COULD defeat Naruto have probably already been mentioned.
Madara, edo or otherwise. Even without invulnerability and infinite chakra, he still probably has a good chance.
Not sure why everyone is saying Nagato even stands a chance. Naruto beat him with just SM, so KM Naruto should wipe him. Even in edo form, which isn't really fair anyway (more on that later). I mean, it wouldn't be an easy fight, but still.
Kabuto in his prime. SM, and with all his summons (madara being one of them). Probably has the best chance, actually. In fact, Kabuto with all of his summons focused on one target is probably unbeatable.
Tobi is kinda questionable. Sure, he is holding off the combined force of Naruto, B, Kakashi and Gai, but thats defense. In a 1v1 with Naruto, I'm not sure if he has the kind of offense to win. We haven't seen such, anyway.
EMS Sasuke. Just no.
Itachi though, maybe. Naruto is weak against genjutsu, and Itachi is probably the most powerful user of genjutsu. Even Karuma might not be able to break Naruto out of some of Itachis techs.
I'll toss some in from left field: Kisame or Zetsu. Mainly because they both have the ability to absorb and integrate an opponents chakra. In which case, Narutos vast reserves actually become a detriment. Kinda unlikely, but worth mentioning.
One last thing about edo summons. In all seriousness, even weak people, in edo form, would pose a problem for Naruto. As far as I know, he has no way to seal; and without that against an edo summon, there isn't really any way to "win". So, I would say that Edo summons shouldn't count for this discussion. Except for in the case of Kabuto, because they are used as tools in his case. Naruto wouldn't need to beat all of them individually, just find a way through them in order to kill Kabuto. Which is still probably impossible anyway.