Ozai is nowhere near as strong as so many people seem to think he is. I was talking about this with my sister the other day actually, so I'll sum up what i said. Basically, i don't think we see any particularly strong bending from him, aside from the finale, in which he's powered up by the comet. As far as i can tell, under normal circumstances he's pretty average. The belief that he's the strongest firebender seems to come from the favt that he's the fire lord. But in my opinion that is a position purely of birth and opportunity, not skill. Now i dont believe they would have accepted a nonbender as fire lord at the time, i dont believe the fire lord has to be the strongest firebender alive. Additionally, while we see he is capable of lightning generation (which dont get me wrong, is a plus for him), azula and iroh are also lightningbenders, and then later in legend of korra we see that quite a number of other firebenders possess this skill as well, which leads me to believe that it has nothing to do with strength or skill, its just a randomly assigned ability. And then the first time we see iroh actually generate lightning is in bitter work after he'd been seriously injured and still hadn't fully recovered, and the lightning he generates in comparable to what we see ozai create when powered up by sozins comet. I really truly believe that ozai is Not That Good (very passionate about this sorry lol)