I voted for the Hurricane. This aircraft ensured that the RAF went to war in a monoplane - the Battle of France would largely have been fought by Gladiators and Furies otherwise, with results best left unimagined. It was also the workhorse of the BoB, although the sexier, faster Spitfire got most of the glory. And finally, it was infinitely adaptable in ways the Spit wasn't, serving as a fighter, night fighter, fighter-bomber, tank buster, and even being catapulted from CAM ships in the Atlantic (making the first Allied a/c available to protect Atlantic convoys in deep water), as well as serving the Belgians and the Russians. Not bad for a plane which represented the final evolution of a line that started with the Hawker bi-plane fighters of the early 30s...