During the comet?
Nobody, except maybe Jeong Jeong.

Maybe Iroh as well, but I wouldn't really count on it.
Ozai is much more of a warrior than Iroh is IMO, Iroh is the kind of guy who can easily exploit the weaknesses of lesser opponents, but I think he would have a harder time against someone with an equal skill level, there would be no easy exploits and I'm not really confident in Iroh's abilities in a prolonged brawl.
There is 1 possible easy exploit though, if Ozai shoots lightning at him, but other than that I don't think Iroh could win.

Without the comet, it gets a bit more competitive, Toph might be able to take him on, like you said, Bumi as well.

Yeong Yeong would still have a chance as well.

I think that Iroh would have a better chance too, without the comet.
I think that the decrease in raw power would better suit Iroh's fighting style.

Katara might have a chance too, but I've always found her power level to be kind of inconsistent so it's hard to really analyze.