Just a question; what do you mean by worst? Worst in terms of contemporary measured performance? Worst in terms of perception by the pilots that flew them? Worst in terms of historical reputation? Worst in terms of the manner in which they were used? Worst in economic terms? Worst to maintain? There is no one aircraft in the greater than 500 operational units category, at least in terms of usage in WWII, that could fulfill all of the above.Which is the worst mass produced (>500 units), monoplane, single-engine, single-seat, retractable undercarriage fighter of WW2? By "mass" produced I'm setting a limit of at least 500 units, so no Vultee Vanguards and its <150 units. The Buffalo stands out, but the Finns did very well with theirs.