Actually, if you go with the OP description no piston engine aircraft even enters into the discussion.
As an example: The XP-80 first flew in January 1944, but all the bits and pieces for the aircraft were available in 1943. The technology existed to plumb wings for drop tanks, doesn't matter if any specific aircraft had the tech installed, all that matters is that it existed. You now have a 500mph level flight fighter with 1,200 mile range (i.e. UK to Berlin and back). As far as I know that is the fastest and best range aircraft using a first Gen engine (in this case the 2,300 ft/pound Goblin 1).
You can also build an aircraft from a clean sheet of paper. The Germans, and independently, NACA had figured out that swept wings would be superior at high speeds (Mach 0.9+) with the Germans having performed much more wind tunnel testing. Both had come up with the ideal 38-45 degree of sweep, neither country USED the knowledge, but it existed, as did the tech to construct the airframe. Build your own design.
As I understand the OP, this is not meant to be a discussion of the best fighter that did exist in 1943, but the best that could be made with the available tech.
As an example: The XP-80 first flew in January 1944, but all the bits and pieces for the aircraft were available in 1943. The technology existed to plumb wings for drop tanks, doesn't matter if any specific aircraft had the tech installed, all that matters is that it existed. You now have a 500mph level flight fighter with 1,200 mile range (i.e. UK to Berlin and back). As far as I know that is the fastest and best range aircraft using a first Gen engine (in this case the 2,300 ft/pound Goblin 1).
You can also build an aircraft from a clean sheet of paper. The Germans, and independently, NACA had figured out that swept wings would be superior at high speeds (Mach 0.9+) with the Germans having performed much more wind tunnel testing. Both had come up with the ideal 38-45 degree of sweep, neither country USED the knowledge, but it existed, as did the tech to construct the airframe. Build your own design.
As I understand the OP, this is not meant to be a discussion of the best fighter that did exist in 1943, but the best that could be made with the available tech.