My choices, based mostly on aesthetics:
WWII
Fighter:
Chance Vought F4U Corsair - Col. Greg "Pappy" Boyington, nuff said
runner up:
Curtiss P-40 Warhawk
Bomber:
Douglas A-26 Invader - Featured in the movie
Always by Steven Spielberg.
runner up:
Boeing B-17 Flying Fortress
Transport:
Douglas C-47 Skytrain - Were there any other transport aircraft in WWII?
Reconaissance:
Consolidated PBY Catalina - The most beautiful thing in the world to a downed air crewman floating in the ocean.
Modern
Fighter:
Grumman F-14 Tomcat - Even when it's on the ground it looks like it's gonna jump up and grab you by the throat!
runner up:
McDonnell Douglas F-4 Phantom - Living proof that you can make a brick fly with a big enough engine.
Bomber:
Rockwell/Boeing B-1 Lancer - Sleek and deadly
runner up:
Boeing B-52 Stratofortress - Have you ever known one of those 70+ year old guys who worked hard all their lives and could still kick anyone's ass?
honorable mention:
North American Aviation XB-70 Valkyrie - The most beautiful bomber ever
Transport:
Lockheed C-130 Hercules - Goes almost anywhere, does almost anything you ask of it.
Rotary Wing:
Sikorsky UH-60 Black Hawk - Looks like a real aircraft, not a toy on a string.
1st runner up:
Sikorsky CH-54 Tarhe (Skycrane) - For when you want to show people you're serious about lifting stuff.
2nd runner up:
Bell/Boeing V-22 Osprey - Very cool!
Reconaissance:
Lockheed SR-71 Blackbird - The most bad ass thing in the air from first flight to last.
Foreign:
Avro Canada CF-105 Arrow - A very pretty fighter, very advanced for it's time. Canceled by the Canadian government while in flight testing. Lots of
controversy and conspiracy theories in Canada to this day. Almost no one in the U.S. has ever heard of it.