The worst was a beat-up and rusty rental bike that I rode for a couple days when we rented a seaside house down in Virginia. It held together just enough to get you to point B a couple miles away, but that was about it. I was surprised it did not break down during the few days I had it. Runner-up would be my younger brother's Wal-Mart Roadmaster mountain bike. Everything was cheap on it and it needed constant fiddling to keep the brakes working in any acceptable way.