Probably the BBMF Chipmunk which is still a working aircraft, earning it's living in its original training role (as opposed to the BBMF display aircraft). As for front line types, I agree, some of those Nimrod airframes are knocking on a bit.
Does Martin Baker still operate its Meteors out of Chalgrove? Thinking civvy for a moment, there are still many DC-3 Dakotas that still turn a profit commercially, and do so in style.