I really don't have a single favorite, but these are at the top of my list of all-time favorites:
WWII aircraft
Curtiss C-46 Commando
Douglas C-47 Dakota
Boeing B-17 Fortress
Consolidated B-24 Liberator
North American B-25 Mitchell
Martin B-26 Marauder
Boeing B-29 Superfortress
Supermarine Spitfire Mks. I, V, IV and XIV
Lockheed P-38 Lightning Interceptor/Fighter/Bomber
Curtiss P-40 Fighter/Bomber
Republic P-47 Thunderbolt Fighter/Bomber
North American P-51D Mustang Fighter/Bomber
Northrop P-61 Black Widow Night Fighter
Republic XP-72 Superbolt experimental Interceptor/Fighter/Bomber
Lockheed P-80 Shooting Star
Japanese WWII aircraft
Nakajima KI-84 Frank
Kawanishi N1K-2 George 21
Kawasaki KI-100 fighter
Mitsubishi A7M2 Reppu fighter
Kyushu J7W1 Shiden
Nakajima Kikka jet
Mitsubishi KI-67 Peggy bomber
German WWII aircraft
Messerschmitt ME-262
Focke Wulf FW-190D Dora
Focke Wulf TA-152
Gotha GO-229 Flying Wing
Junkers JU-87 Stuka
Arado AR-234 Blitz
Post WII
Fairchild F-119 Flying Boxcar
Douglass C-124 Globemaster II
Boeing B-47 Stratojet
Boeing B-52 Stratofortress
North American F-86 Sabre
Lockheed F-104 Starfighter
Republic F-105 Thunderchief
Convair F-106 Delta Dart
McDonnell Douglas F-4 Phantom II
McDonnell Douglas F-15E Strike Eagle
Lockheed Martin F-16C Falcon
Rockwell B-1B Lancer
Northrop B-2 Spirit
Lockheed Martin F/A-22 Raptor
Lockheed Martin F-35 Lightning II
Jim