[valkyry1]

For me it would be the Republic XP-72, but the war ended before it went into service.

It was based on a advanced version of the P-47 incorporating the Pratt & Whitney R-4360-13 Wasp Major 28-Cylinder radial air-cooled engine producing 3,450 hp.

It would do 490 mph without the supercharger turned on, top speed was estimated up into the 550 range for the prototype using the contra-rotating props.

In fact there were several fighter offerings to be based on the awesome Pratt & Whitney R-4360 engine. Several of them were to incorporate contra-rotating props to absorb the immense power and torque of the engine.

http://www.warbirdsresourcegroup.org/URG/xp72.html

http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=419

http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?fsID=2272

[GonzoGOP]

If you are counting experimental/limited production they the P-80/F-80 comes into play. They actually got some of them to Europe, but they never mixed it up with the enemy. The thought of P-80s vs ME-262s in the sky over Germany is just too cool to imagine. Heck throw in the Horten flying wings and just imagine the fun.

[mikeus_maximus]

That... would have been a game-changer.