I say the Christmas Bullet, an airplane claimed by it's designer to be the "fastest" and "safest" in the world.
![Mad :x](./images/smilies/icon_mad.gif)
It was designed so that it's wings would "flap", like a bird's, but in actuality, they merely broke off shortly after take-off! There is speculation whether or not Dr. Christmas got his doctorate from a mail-order diploma mill. After killing two army test pilots with this monstrosity, he successfully got re-imbursed for thousands of dollars, and died an old man, with money in his pockets.
Criminal.
Others in the Really Bad list include:
The Caproni Ca 60, a NINE winged houseboat that got about 20ft into the air before breaking up. And Count Caproni built such good planes,normally.
The Yokosuka MXY-7 Okha, unlike the Chrstmas Bullet, the pilots of this human guided missile knew it was designed to kill them.
The Bachem 349 Natter, another pilot killer that, thankfully, never saw service. Look familiar, Wing Commander film fans???
Any more?