IMO it's the [url=http://en.wikipedia.org/wiki/Sukhoi_Su-47]Sukhoi Su-47 [i]Berkuit[/i][/url]. It's a shame they only built four of them.
Some honourable mentions: [url=http://birdsnbugs.files.wordpress.com/2012/03/dsc_2782acr107.jpg]MiG 17[/url], [url=http://upload.wikimedia.org/wikipedia/commons/5/51/Grumman_F9F-2_Panther_in_flight_c1949.jpg]F9F Panther[/url], [url=http://oi57.tinypic.com/25rgww0.jpg]Horten Ho 229[/url], [url=http://latimesphoto.files.wordpress.com/2012/10/fa_396_goose47flies970.jpg]H-4 Hercules AKA "The Spruce Goose"[/url] This thing has a wingspan of 320 ft 11 in (97,54 m)! It only flew once and it's now in the Evergreen Museum in Oregon.
What do you think is the best looking airplane ever made?
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Another honourable mention; the F-82 Twin Mustang.