My opinion is that for the early part of the war, the most significant air battles were at sea. I would rate the disabling of the bismarck, the Battles for malta, and the disabling of the italian fleet at taranto as the most significant actions until the entry of the US. The reason is that it was at sea that had the greatest potential to cause an upset in the war. If the Axis could get heavy units into the Atlantic, they could threaten the escort forces holding the U-Boats in check. If the escorts were neutralized by these heavy units the war was essentially lost.
In the med, this seemingly secondary theatre had the potential to cause an upset. The key to the MTO was logistics, and the key to the logistics was malta, and the key to malta was the control of the oceans and the skies around it. If the British had been unable to undertake force projection into the central basin, (and the key to doing that was the carriers providing support to the RN), Malta would fall. with Malta fallen, the Axis supply problems to the North African theatre would be solved. With logistics solved, the Germans would almost certainly have overrun the oil fields of the middle east. With these under their control, the whole balance of power would change.
The actions by the miniscule CAGs onboard those handful of RN carriers (and on malta....an unsinkable carrier essentially) changed the course of history........