I'll go with the P-40. Woefully outclassed in both theatres at the outbreak of the war but deadly in the hands of a skilled pilot. Take a look at the kill ratio of the AVG in China against the Japanese Zero....
WWII bomber pilots get nothing but respect from me. If you think about what they were doing and the fact that most of the "old men" of the squadrons were in their early twenties. How many twenty year olds would you trust today with the keys to the Liberator?