It's a tie, between two lenses, for the biggest piece of chit zoom lens EVER foisted on the buying public. The first was a long time ago, and was a Zoom~NIKKOR 35-135mm Ai-S...I believe it was an f/3.5~4.5. Oh-My-Lord...wow...what an utter DOG it was. I bought it from my friend Michael...who had bought it from a friend...who had bought it from a friend...cough,cough. NOT sharp, low contrast, utter rubbish. This was about 1985. Man, that lens SUCKED. I would say it was the worst lens I had in the first part of my life.
Fast forward to 2001. I bought the Tokina AT-X Pro 28-80mm f/2.8 zoom. It quickly became known as "Der Flaremeister". And to make matters worse, I bought it brand-new, at full retail, which at that time was like $699 or something ungodly. It had a serious problem with light sources. If you even THOUGHT ABOUT shooting it toward the sun, or the bright sky, it would preemptively flare, as a way to make you reconsider the upcoming shot!