I gotta go with Kursk.
Battle was anywhere from 10 days to a month and a half, (German view, Operation Citadel 10 days, Russian view, battle lasted till Charkow/Kharkov fell).
At least 2500 Soviet planes, 3000+ if you count units kept in reserve or on less active fronts
A little over 2000 German planes.
Each side flying as many as 4500 sorties in a day.
Kursk is heralded as a tank vs tank battle, but some of the most important sectors were won by air vs tank. It was the proving ground for the Soviet tactical air/ground cooperation concept, which eventually took them all the way to Berlin.
BoB was important too, but Kursk surpasses it (and probably all others) in scale.
In my opinion Kursk was the greatest air battle of WWII, for numbers of aircraft involved, number of sorties flown, and importance as the pivotal battle on the Eastern Front.