It is hard to argue that the P40 was the best or most significant fighter bomber of WW2. (and impossible for the Zero or CR42) However it can be argued that the P40's use in the Middle East served as a prototype for what followed in the ETO and the Pacific with more powerful and advanced fighter bombers such as the Typhoon and P47 continuing the dual use role pioneered by the P40. P40 units such as No 112 RAF were amongst the first to successfully carry bombs to supplement their guns in the ground attack role, as well as continuing their fighter role and escorting bombers.
And equally importantly the systems and communications to control "fighter-bombers" in this role were also pioneered in the desert.
If aircraft can't be got to the correct place at the correct time to do something useful then the whole concept fails as it did for the RAF's AASF in 1939/40.
Cheers
Steve