Admiral Beez
Major
Excluding air cooled V12 fighters like the Caudron C.714 or fixed undercarriage fighters like the prototype-only Miles M.20, which were the worst liquid-cooled, V12-powered, single-engine monoplane, retractable undercarriage fighters of WW2? IMO, contenders would include the LaGG-1, LaGG-3 and Rogožarski IK-3. I'm less sure about the Heinkel He 100 or Reggiane Re.2001. While not a production aircraft, the Kawasaki Ki-60 was reportedly deeply flawed, but led to the competitive Ki-61.