I like the P-39. I don´t know if it would make my top five list, but it´s a pretty cool aircraft.
My top five favs :
1. Spitfire Mk.XIV ( just the sound of the Griffon engine is reason enough to place it first ).
2. B-24J Liberator ( big, bad and ugly. How can you not love it ? )
3...
The best WWII biplane fighter was probably either the Gloster Gladiator or the Polikarpov I-153. The latter was far superiour to the CR.42 and also had the edge on the Gladiator with its retractable landing gear and far better armament.