It's hard to say, really. Sinse we've never seen them fight eachother. They did say Iroh was the only one who could possibly defeat Ozai besides Aang, but I think even Iroh had doubts. Ozai's firebending is also powered by hate, and Ozai wouldn't have mercy, while Iroh believes in mercy. Ozai is also suppose to be the strongest firebender in the series- next coming Iroh and Azula, possibly Zuko. I do believe Ozai is the strongest in a sense. His power madness also causes him to be cruel.

Iroh does have wisdom on his side, though. I know Ozai is a smart person, but Iroh knows more about balance and whatnot, while Ozai just doesn't care as much.

Ozai is also able to generate lightning faster than anybody else but Iroh can re-direct it which might make them even then but Iroh would need to be prepared for an attack as powerful as that and Lightning isn't slow.
I'd say they are both very powerful benders but Ozai just about gets my vote. Debatable though.

It's verry hard to choose between them, cause Zuko said to his uncle that if Aang won't fight Ozai (cause he was missing), Iroh is the only one who can face him and kill him. But Iroh admited himself that even if he is verry powerfull, his brother Firelord Ozai might win.

Both.

By far.