1, Spitfire/Hurricane(mention is despatches to radar and the observer corps)
inseparable as they decided the outcome of the war for the Western allies, If the Luftwaffe had neutralised the RAF in the BoB, Britain would have either had to negotiate peace terms, if Hitler was prepared to, or surrender or risk invasion!
either way, with Britain and the commonwealth out of the war in Europe there would have been no big week, no bomber offensive, no defeat of the Luftwaffe, no P38/47/51 in Europe or the Med, in fact would the US even have entered the war?
politically we could easily have seen WW2 diverge into separate European and Pacific conflicts, and ultimately Europe could well have been a communist empire as I believe the Russians would have defeated Germany in the long term, unless of course the Germans produced a viable nuclear weapon first?
So first I would have to say the Spit and Hurricane, simply because by keeping Britain in the war they set the stage for everything that happened next, without them the world could well be a very different place to what we know today!