Okay then. Here's my list - bear with me. It covers all three of the armed forces' air assets in six airframes for each of the three services; the distinctive role air power plays within each service being easily discernable. The aircraft are selected based on their entering service by the end of 1940 at the very latest, but following the original stipulation that each type must be chosen for service throughout the rest of the war.
Air Force:
Spitfire: interceptor, strategic photo recon. Tough choice between the Spit and Bf 109, but the Griffon engine tipped the scales in favour of the Spit.
Wellington: heavy bomber, HCU/navigation trainer prior to 1940; after then the Manchester, which led directly to the Lancaster.
Ju 88: medium bomber, pathfinder, night fighter. Either Ju 88 or He 111 equipped with Knickebein/X Gerat/Y Gerat. The accuracy offered by this equipment was second to none in 1939 - 1940.
Beaufighter: close support, maritime strike, tactical recon, night fighter. Touch choice between Bf 110 and Beaufighter; the former had better performance, but the latter had a better weapon load for maritime strike and a better observer's position.
Hurricane: close support/anti-tank, night fighter.
AT-6: trainer, liaison.
Navy:
Kawanishi H6K: long range maritime patrol flying boat.
LB-30 Liberator: land based maritime patrol, transport.
Arado Ar 196: ship based recon.
Mitsubishi A6M: carrier based fighter.
Nakajima B5N: carrier based torpedo bomber, recon.
SBD: dive bomber, recon.
Army:
Bf 108: liaison, transport.
Mitsubishi Ki-46: recon, medium transport.
C-47: heavy transport, glider tug.
Fw 200, long range passenger transport.
Fi 156: aerial observation, liaison.
DFS 230: assault glider.