Hello Oreo
a good and very difficult question. I'm not able to choose the most underrated.
P-39, not the greatest fighter but definitely better than its reputation. Even Ds didn't do so badly than seemed to think and Ns and Qs did rather well in SW Pacific, but they lacked range and after going offensive US needed longer ranged fighter there.
With Soviet AF, now the bird was successful, many of the Soviet top aces flew them and the CO of 9th GIAD, Pokryshkin, refused the offer to chance Cobras of his division to the newest Soviet type. And one big offensive was delayed so that the supply of 100 octane fuel for 9th GIAD Cobras could be arrange before its beginning and Ns and Qs were used by 9th right to VE-Day.
Now F2A, or to be more exact B-229, did a sterling service in Finnish AF and had very light and effective controls and reasonable firepower. Also Dutch seemed to assest their B-339Ds better than Hurricane II with tropical filter.
I-153 and I-16 were both very manouevre and had rather good powerloading. Of course I-16 was an old type in 1941, but when it entered service it was a very modern concept. I-153 was on other hand a dead end when accepted in service.
Bf 110 was definitely better than its popular reputation in English speaking world indicates. A truly multi-role a/c and even if did not live up Göring's expectations during the BoB it wasn't a bad plane.
P-36/Hawk 75A/Mohawk, the most successful fighter of the French AF in 1939-40, did a sterling service in the Finnish AF 1941-44 and was still useful in Burma in 43-44 or 1944 only against Ki-43s which also were better than their reputation. As Spit pilots found out, Ki-43 wasn't a plane to underestimate and in no way an a/c to be fought against in slow speed turning combat.
Maybe I'll make my choice later.
Juha