10?!?!?!? All I get is 10??!?!?? How do you narrow it down to 10??? There were so many planes that were so important for the history of aviation that it is really difficult. I agree with the J-3 as important, but how about the thousands of pilots trained in the Cessna 150/152 series or the PA28s?
I tried to stick just to U.S. aircraft (not that I don't recognize such great military machines as the Spitfire, Hurricane, Sea Fury, Me.109, FW190, etc) and found it really hard to think about. Some of the most significant civilian birds:
North American/Ryan Navion
Beech Bonanza
Lockheed Constellation
Lockheed L1011 TriStar
DC3 and its many permutations in the military
The Boeing 247
Beech 17
Beech 18
Boeing 707
Boeing 727
How about the SR-71 for a very significant bird!
When I think of attack aircraft, I end up with
Douglas Invader
Douglas Skyraider
Douglas Dauntless
Douglas Skyhawk
Cessna Dragonfly
Fairchild-Republic Thunderbolt II
Bombers
B-17, B-25, B-29, B-36, B-47, B-52, B-1, but don't forget the PBY Catalina
but very significant in the development of the B-2 was the B-35 and B-49
Transports (not counting the C-47 which was included with DC3)
Curtiss C-46 Commando
Douglas C-124 Globemaster II
Lockheed C-130
Boeing KC-135
Lockheed C-141
Lockheed C-5
Trainers would have to include
AT-6/SNJ Texan
Stearmans (PT-13/PT-17/PT-18/N2S)
North American T-28
Lockheed T-33
Northrop T-38
My real problem is deciding about fighters, since so many made history for various reasons. I just can't get near 10 there
Curtiss P-6
Boeing P-12
Boeing P-26
Lockheed P-38
Curtiss P-40
Republic P-47
North American P-51
Northrop P-61
Lockheed P-80
Republic F-84
North American F-86
North American F-100
Republic F-105
Convair F-106
McDonnell F-4
Vought F-8
Grumman F-14
McDonnell-Douglas F-15
Grumman F4F Wildcat (incl GM built)
Grumman F6F Hellcat
Grumman F7F Tigercat
Grumman F8F Bearcat
Vought F4U Corsair (and FG models)