Just for my own personal interests, I'd love to see the G.55, the G3M Nell, and the Bf109E-7.
Outside of that, most useful for AH would be an He111 (also can sub for other nations' early bombers), Ki-21 Sally, Ki43 (say what you will, it was the primary IJA fighter for years of the war and we are missing ANY equivelant substitute), and yes we could always use soviet planes like the Pe-2 or Italian planes like the SM79 or C200, but generally speaking early war bombers with minimal defenses and slow speeds are vital to every FSO or scenario we could run. The way bombers run at top speed in this game makes them very hard to catch with early era fighters, which wasn't the case historically. To this end we need a much much more fleshed out early war bomber set across all nations.
BOB is rather a joke with our Ju88A4s. Hurricanes can't catch them unless all conditions are perfect. Making a few degree variation in course means you cannot catch them. This wasn't the case in the BOB. Prolonged attacks with multiple passes were the norm. That is why the He111 is needed. The BOB is one of the most beloved scenarios in the game. It is a universal attraction that can't be run very often because we just don't have the planeset for it.
Anything in the Pacific absolutely needs the Ki-43. It was so nimble if it ever latched onto you, you were in trouble. This is tempered by a lower top speed and a smaller gun loadout. This was the primary plane fighting any allied forces for years in the PTO. The A6M2 zero was relatively scarce outside of carrier battles. It was the Ki-43 fighting everything and might I add holding its own.
Ki-21 Sally is just an early war bomber to help flesh out the early war bomber planeset. You could substitute the Bristol Blenheim, or the SM79 or a number of early war bombers with comparable light bombloads. These could be used as substitutes for each other in a number of setups. The Ki-21 allows us to run a number of early PTO setups (along with the Ki-43) but could also substitute for a He111 in the battle of britain, or an IL-4 on the soviet front, or a number of bombers on any front you can imagine.
Those are the 3 most needed and the reasons why. Or count it the 2 most needed and the other category.