It depends on what you mean by "underdog". Most fighters are underdogs when compared to the P-51D, Typhoon Mk Ib, Fw190D-9, Bf109G-10, La-7, Yak-9U, N1K2-J, Spitfire Mk IX and P-38L.
If we mean underdog to those, then my favorite is the Mosquito FB.Mk VI.
If we mean slow assed, massively undergunned 1940 fighters as underdogs, then I'd pick the Spitfire Mk Ia and A6M2.