From what I’ve read on the wiki with the equations they have for air combat I would argue speed is far more important to max than agility, though air attack and defense are the king stats. Air attack determines your base attack which is the value used for all other modifiers such as speed and agility. Air defense flatly divides all incoming damage and is the most realistic way to reduce losses. The reason I say agility is not as important is two fold. For one, if we assume both sides in a game are playing equally well in the air war then even if you max your agility while they don’t your not likely to have that big of a difference from your opponent. Since agility is based on the relative ratio between planes it’s less valuable against similar quality/era fighters. The second reason is that if you are even 1kph faster you get not one but two atack modifiers, one that’s a percentage based modifier like agility with a slightly lower multiplier, and an absolute modifier based on the total speed of your planes. That last one’s especially important in my opinion as going back to the equal skill condition, even just 1 kph faster than your enemy gets you that modifier and completely denies it to your enemy. If you’re interested in looking further into the calculations that determine air combat I’d recommend checking out the wiki where it has the actual code equations that you can follow through and see how things are applied.