One could cite anything manufactured by Blackburn throughout their long history as far as British aircraft are concerned. How did this company get so many contracts, which would be laughable if those contracts didn't result in injury and loss of life? However this company did eventually produce an outstanding airframe, the 'Buccaneer'.
So overall worst aircraft in WW2 of the Allies has to be (for me) the Blackburn Botha, having many problems, the worst of which it couldn't get off the ground carrying more than a fraction of it's payload!
I was surprised to see the Ju87 and Swordfish in the last list posted, considering the success they had in their roles throughout the conflict.
As to the Defiant, due to the unimagined quick defeat of France, it was forced into a combat role it was not designed for, causing great attrition of the courageous crews that continued to fly it in the Battle of Britain, until it was withdrawn from that conflict.