its really hard to say because Zira was dominating the fight but in the lat moment it was all about Simba and Zira.

When it came right down to the one-on-one fight, Zira. Simba was already hurt and that would work to her advantage.

I agree with Okami_Amaterasu, Simba did seem quite weak and wasn't really in a good state to fight. Zira hadn't been fighting at all, so she had the strength and energy for that fight.

I want to say Simba, but what was shown of the battle made it seem like Simba and his pride had underestimated them, I doubt Simba would have won the fight.

I think it would be Simba, because he's a male lion. And he's ten times larger than Zira, Simba is much stockier and built, and Zira is slimmer and smaller. Especially Males are much powerful than females,
females can only defeat a male in groups. Besides Zira could end up
wounded and injured by Simba in real life, if they're alone without their prides.

Simba because he's much larger and stronger, plus he's not a starving lion like Zira. A male lion can kill a female very easily by one and one. I seen it before on discovery channel. males are much brutal and heavier than females.

It probably would have been close but Simba would have been victor

Simba although weakened. In the stampede from the first film Mufasa was able it jump and grab Simba after the weldabeest kicked him and climb the cliff after being knocked back into the stampede.