The Mossie was terrific, but I think the Spitfire edges it out because it was the only plane that was top class both at the start of the war and at the end using largely the same design.
The Spitfire was designed as a short range, high performance air superiority fighter and every incremental upgrade and design tweak over its life was to keep it at the top of the tree as such, not turn it into a long range escort.
When the war had progressed to the point where long range escort fighters were required, the Americans had arrived with a Merlin powered P-51 which was designed from the outset with much bigger fuel tanks and it's airframe was better suited to adding drop tanks and squeezing in extra fuel tanks.