Here are 10 Under Rated i have quickly thought up, probably some glaring omissions -
1) Hawker Hurricane: Hawker Hurricanes flew in every theatre of WWII and flew operational sorties on every single day of wartime up until VJ day - most seem to think its career was over after the BoB, but it served fantastically in Malta, then protecting convoy's and off of carriers and finally as a ground attack aircraft, especially in North Africa and then Burma right up to the end.
2) De Havilland Mosquito: As a low level attack aircraft, pathfinder, night fighter and photo recon aircraft it excelled, pretty much the eyes of the allied air force along with the PR Spitfires.
3) Hawker Typhoon: Often overshadowed in American aimed books and film by the P-47, the Typhoon cab ranks decimated the Wehrmacht and was an awesome fighter at low level.
4) C-47: No need to explain.
5) B-24: Often overshadowed by the B-17.
6) F6F: more IJAF/IJN kills than any other aircraft?
7) Hawker Tempest: Especially under appreciated by the British public, the Tempest shot down an incredible amount of V-1's before they could kill civilians, which is often overlooked.
8) Fairey Swordfish: As already mentioned, succeeded in outlasting nearly all of its planned replacements. Showed that capital ships (much to the japanese's interest) could be attacked in port, 20 or so single handedly kept the Italian fleet out of the mediterranean after the attack at Taranto.
9) Wellington/Warwick: Decent Bomber and fantastic service in the Battle of the Atlantic.
10) All US Medium Bombers: Always overshadowed by the heavies.