My vote on worst bike goes to the Yamaha XS650 Special. Would never start on the button, always had to kick it. It would run fine one day and lean surge the next, and then the following day it'd load up and run rich. Replaced every part in those carbs to no avail. Electrics were junk. Wires are mostly aluminum and corroded, switches were poor quality and only worked part of the time - even after disassembly and cleaning, breaks were lackluster at best, and the bike had a dangerously nasty weave at 80mph. Couldn't keep rear swingarm bushings in it, they're *bakelite*. Not even plastic. You can get bronze bushings now, but you couldn't 13 or 14 years ago when I owned it. Oh, and it vibrated so badly the points wouldn't stay adjusted from the house to the end of the driveway. The mechanical advance unit would bind up at full advance and not return to center like it should - even after replacing it, it would still do this from time to time. Pull chrome cover, pinch it closed, and then the bike would start. it went through light bulbs like it was cool, and the headlight was so dim it was useless - even after replacing it with a halogen kit. That bike gave me NOTHING but trouble.
Only bike I know that came from the factory with a reverse gear. Put it on the center stand and rev it to 4 or 5k and it would vibrate backwards, uphill, at a walking pace.