Some good ones already mentioned, but I agree with Bob Viking that several nominated may rate very high on the nostalgia scale, but perhaps not so much on the usefullness side.
I agree with the A-10, OV-10, Buffalo/Carribou, A-4. I also nominate a turbo prop Skyraider for COIN work and a fresh F-15 with the latest avionincs.
I respectfully question serveral nominations so far, while again they may be high on the nostalgia scale, even with a major upgrade some inherit design limitations may limit utility:
-Vulcan- Yes very sound design, but high radar cross section. Fine for a standoff launcher.
-Buccaneer- high radar cross section.
-Lightning- poor range, manueverabilty nowhere near later generation fighters due to wing design. New engines would help. See it as point interceptor only- not multi-role.
-Harrier- mixed thoughts. Yes a sound design that would greatly benefit from a refresh, but nozzle layout makes it vulnerable to IR threats. Great when you really need V/STOL, but how often do you REALLY need V/STOL? And no I don't want to start a Falklands debate.
-Canberra- for what? As a medium bomber I think it would be very vulnerable in todays threat environment.
-Concorde Bomber- yes very sexy looking, but do you really need that speed and height to launch cruise missles?
-JAG. Mixed thoughts here as well, yes an upgade would help, espcially with better engines, but payload was limited.
-Gannet- I think other designs have more to offer (E-2, S-3)
-ducks for cover