Oh, let's talk beautiful planes :)
WWII-era: Me 262 takes the top spot with its shark-like fuselage and sleek lines, the P-38 as mentioned looks simultaneously both powerful and really, really graceful, the Westland Whirlwind is also gorgeously aggressive with its propellers sticking out in front, and of course the old stalwarts the Spitfire and the Mustang are magnificent designs. Then there's the striking utilitarianism of the Me-109 and the robust stubbiness of the early Fw-190, and the impressive gaping maw of the Typhoon. Also the P-48 Thunderbolt has a certain rotund charm, wouldn't you agree? :)
Moving on to the cold war era, the SR-71 is the undisputed queen of the air, beautiful as can be and as much a work of art as it is a plane, but the B-58 Hustler is the epitome of a jet bomber, with the B-47 a close second. For fighters, the MiG-29 and MiG-31, the F-14 and F-15 are planes I grew up loving, and the Swedes did all right with their beautiful delta-wings, the sleek J-35 and the more angularly powerful J-37 (which incidentally was the only aircraft that ever managed to get a radar lock on a SR-71 - and they did it on numerous occasions).
Current planes, well... I'm not a fan, really. Of the current crop of fighters, I guess the Swedes did it again with their J-39; I don't really like the F-22, F-35, Typhoon, or any of the Su-27 variants that seem to number in the dozens across Russia and China. It all seems to be drones, drones, drones anyway these days, and I sometimes long for a past where planes weren't designed by computers but by eye and hand.