Favorite? One? That's so hard. There are so many neat aircraft. Spitfire, Me410 Hornisse, A6M Zero-Sen, Mosquito, B-17 Flying Fortress, Lancaster, ect, ect.
I can't narrow it down to one, but I can do two, painfully.
Spitfire F.Mk XIVc
Beautiful, fast and deadly. What more can you ask for. It is the most beautiful warplane ever built (IMO) and it still looks the part. As close to perfect as any WWII aircraft really got for air-to-air, fighter-to-fighter combat. The angry growl of a hungry Griffon, the stacato of twin long barreled Hispano cannon, the aircraft as an extension, as a part of the pilot. The very essence of a fighter.
Mosquito FB.Mk VI Series 2
The epitamy of versatility. Wave-top, tree-top, roof-top level flights at speed, 345mph on the deck. Gestapo killer. Elegant lines and curves, pretty stepped leading edges on the wings. Such clean lines, nearly unmarred by seam lines or panels yet tough and durable, damage absorbing wood. Machine guns, cannon, bombs, rockets...teeth. Twin roaring Merlins, 3,270hp rippling from the engines into the air. Vengeance. Striking against the aggressors.