Overrated: P51
Underrated: Bf109
I'm with Slurpee on this one. The 109 was upgraded countless number of times and an airframe originally designed for a 600 hp engine mounted by the end of the war a 2000+ hp engine! It was not a high speed design yet incredibly competitive in the air, the germans biggest problem was not the machines, it was the men supposed to operate them there were none left.
Perhaps the 190D is a bit underrated, it's hard to tell because there is no hard evidence on it's performance. It's conveniently 'lost' despite both US and SU had airworthy planes of the type post-war. Granted it entered service too late to play any big part in the war, it's performance may be somewhat underestimated, is my guess.