My worst was definitely the racer lookalike I had for my 6th birthday. The cranks couldn't be tightened up properly so they moved around making it horrible to pedal, the bottom bracket was stiff and so was the brake mechanism which resulted in my first proper crash and a dislike of using drop bars ever since.
The next worst was probably my Apollo Excel - a misnomer if ever there was one!
To set the scene, I keep a cheap bike for use in inclement conditions, trips into town and general larking about. My previous knockabout bike needed repairs and tallying up the parts prices I was going to have to spend at least £80 to make it useable again. Wandering round
Halfords I spotted the dual suspension Excel which was on sale for £60. I'd never tried a suspension bike at this point and at that price it seemed a no-brainer so I got it. I never managed to set it up so it was comfortable to ride and the brakes supplied with it were so poorly made they were bordering on dangerous.
Surprisingly I stuck with it for 5 years

(once the V-brakes were replaced) but eventually the drivetrain and rims wore out and the forks developed a worrying amount of play so I had to replace it.
The new one (Hawk Trakatak)
is nice to ride but I've had to replace a lot of parts due to breakages. Can't win them all I suppose.