Ah, I completely forgot about this. Thank you for refreshing my memory ^.^
He may not have been as powerful as Ozai, without an Agni Kai between them it really is impossible to say. However, his knowledge of tactics and human nature would have trumped the bestial rage that Ozai brings to the fight in my opinion.
Another thing I was thinking about (while I was bored in class earlier today) is that Ozai was born in the Fire Nation, raised in the Fire Nation, learned firebending, and became Fire Lord. I doubt he's ever left the Fire Nation except maybe once or twice to conquer something (even then, I would assume he'd use Azula or General Zhou).
Iroh has travelled to all the nations, studied the ways of their people, and has learned to incorporate techniques from all 4 Elements of bending into his own bending, making him that much stronger. Not to mention he's friends with some of the most powerful benders in the world (see Order of the White Lotus).
Firebending, being a naturally aggressive bending style by nature, usually gets trumped by the agility of airbenders.
It reminds me of Zuko and Aang, when Zuko would just go full out on Aang and Aang would just jump or crouch to block it.
Here's the best video I could find of that (although why he didn't just have the sound and not a song, I don't know)That being said, I don't know if Iroh would be as swift as an Airbender at his age and...mass..
I would still say Iroh > Ozai
