I have to break it down by country, and era I think, because Nesher is right.
It's impossible to pick, just one.
Modern aircraft, proven in combat, I'd have to say, the following are my favourites...
McDonnell Douglas F-15E Strike Eagle = United States.
Sukhoi SU-34 Fullback = Russia
Panavia Tornado GR Mk. 4 = Germany, Italy, UK
Dassault Rafale C = France
AIDC Ching Kuo F-CK 1A = Taiwan
Cold War:
F-104D Starfighter = U.S.
Blackburn Buccaneer S.2C = U.K.
Mikoyan Gurevich MiG-25P Foxbat B = Soviet Union
Dassault Mirage IIIE = France
Nanchang Q-5 Fantan = China
WWII:
Supermarine Spitfire Mk.IX - UK
Focke Wulf F.190D = Germany
Curtiss P-40 Tomahawk = U.S.
Mitsubishi Ki-43- Mk.IIIB = Japan
Birds that didn't make it:
Being Canadian, naturally, I have to whine about the Avro Arrow Mk.1
Northrop YF-23A Black Widow II = U.S.