Back to the subject in hand......
I am unsure about how to justify the worst aircraft but one that does come to mind is the Manchester. I understand it to be a good enough aircraft, but the engines were rather unstable...to the point of just randomly catching fire. I cant really add much more as my knowledge doesnt stretch much further and i reallyt cant be arsed at the moment to go look it up...I am sure there are many bad aircraft...it just depends on how you catogerise them....Is it a bad aircraft because it has no speed, or because it is unreliable, or because it cant carry much arms, it may not have much distance. I really dont know!