Well of course the Hellcat and Corsair were superior aircraft, they entered production two years after the Zero. But at the time it was introduced, it was arguably the best fighter aircraft in the world, and inarguably the best carrier-based aircraft. This was the first carrier-based aircraft that could match a land based aircraft in combat, most of those underpowered American planes were top of the line when the Zero was introduced. Its more fair to compare the Zero to the P-39 Airacobra which still was introduced a full year after the Zero, yet lagged far behind it. But the great technological leap the Zero made was in its range. The real limiting factor on long range missions for the Zero wasn't the limitations of the Aircraft, but the pilot. This aircraft could fly almost exactly 2000 miles, completely outstripping any enemy aircraft.
In the initial months of the war, the Allies overestimated Japanese airpower greatly, because fighters could be brought from bases so far away for any one operation, it seemed that they had much greater numbers.