There are definitely spoilers in my thoughts. These are the four options in my mind. Aang obviously as the Avatar, and for the sake of argument, let's disregard the avatar state. He knows all four elements and is very good at them. Ozai is the most powerful firebender, debatably. We can confidently say he is stronger than Azula at the least. This is not to be considered during the comet. Iroh is the one firebender who could potentially be stronger than Ozai. He didn't deny the option that he could beat Ozai in a fight, but didn't think it was likely. What makes me consider him is his feat during the comet, where with its added power he breaks down the wall of Ba Sing Se by himself. I can't guarantee that Ozai can do this, though maybe he could. Iroh is also capable of lightning redirection, meaning his lightning could kill Ozai, but Ozai's would be useless. The Mechanist is a different kind of power, but modern and future warfare are completely dedicated to this man, minus the Firenation's battleships. He invented their tanks, their airships, both weapons used during the invasion, and many more devises used before he's released from Firenation employment. I haven't watched Korra, but I wouldn't be surprised if he designed a good portion of their modern life. Katara is another option, because thanks to Hama, she has bloodbending. During a full moon then, perhaps she could beat anyone on this list. It isn't clear whether or not Katara is bending her own blood when she's able to move before bloodbending Hama in that fight, but if she isn't, then my suspicions would be correct that enough chakra being controlled by a person can disrupt someone else's control of them. This would suggest that the other three people on this list that can control their chakra should be able to resist her bloodbending if they are that much stronger than her. She's also just a very strong waterbender at this point, so maybe they couldn't. This is why there's a poll.