Ok, can't resist on this one...
WWII: Best has to be the Mosquito, although the Spit should get an honourable mention (and, just to be whimsical, I'd also add the local conversion of the Buffalo in Singapore which flew over 100 sorties without loss at much longer range than any of the other local conversion jobs like the Hurricane)
Cold War - Strategic: I'd have to go with the SR-71 'cos it's fast. Second place would go to the Canberra PR9 (there's still nothing that can manoeuvre as well as the Cranberry at high altitude - something to do with having a chuffing great wing!) and the U-2/TR-1 coming in not too far behind in third.
Cold War - Tactical: I think the RAF Phantom gets this vote. It carried an amazingly capable (and bluddy huge) recce pod and was very capable, albeit relatively short-lived in the role. The Jaguar plied the trade long and hard but the aircraft wasn't really a great performer - it is said that on a hot day in hot/high climates, the Jag got airborne because the earth is curved.
Gulf War: Tornado GR1A. Alone and Unarmed. Personal favourite, but then I'm biased 'cos I served on 13 Sqn which, sadly, is going to be disbanded later this year.