I am not sure what was the most overrated plane. On the West possibly it was P-51, in Russia Yak-3.
But I am confident that the most underrated fighter was P-39. What is the tragic fate! It isn't known on the West, and it is forgotten on the East.
At the beginning of the war Soviet Union could set against Bf-109 only I-16 and I-153 which didn't match. Yes, russian pilots did their best but Messers won on the verticals, had speed to disengage when they wanted, and I-16s could do nothing with that. And P-39 became the first fighter which matched German fighters. Literally, it was P-39 who won the air superiority on the Eastern front in 42-43. La-5FN, Yak-3, La-7 saw the war when actually it was over, in late 43 - 44.
But because of ideological reasons it was impossible to admit that P-39 was one of the best fighter on the Eastern front. You know, cold war, american imperialism, etc. For example, when I was at school I knew the name of the triple Hero of the Soviet Union Pokryshkin, but I had no idea that he flew on the American fighter. I thought it was Yak-3.
Pokryshkin - among his 59 victories 48 were on P-39;
For others it is the total number of victories but most of them were on P-39
Gulyaev - 57
Rechkalov - 56
Golubev - 53
Glinka - 50
etc.
