What can I nominate that hasn't already been said?
I always bring up the Flying Tent Peg, but it only really sucks if you treat it like a normal plane, and then you suck all the way down to the ground.
You had to get up, whack something and then go down, if you got into a turning dogfight, well then you're going to struggle.
The Defiant sucked because it was a World War One fighter in World War Two, however it excelled at the night role, so it's not that much of a sucker really...although those who used it after the 109s had twigged on the difference between it and a Hurricane would probably differ in opinion.