Oh sure, there maybe contenders for this category based on safety issues or complexity, but my assertion for the worst toy of all-time is based simply on the mind-boggling ability of toy companies to be able to market ANYTHING to kids. I guess an argument could be made that the even-older "Pet Rock" is perhaps the greatest sham from strictly a marketing standpoint, but Mattel's SLIME of 1976 showed that toy companies could sell absolute shit.
I'm not exaggerating here; for all I know this stuff was the natural discharge of some kind of crazy creature that lived in a cave system in Outer Botswana. Unlike the Pet Rock which you could customize, take with you, and keep in your pocket for Show and Tell, Slime was fun for about 10 seconds until you were thinking "GET THIS SHIT OFF MY HANDS!" There should've been a clue when it came packaged in a garbage can. Perhaps they were appealing to the Oscar the Grouch fan club.
The looks of the kids in the pic above are not those of children enjoying themselves. They're demented. They're confused. Hell, look at the eyes on the first kid - he's Satan! You could not carry Slime in its natural form anywhere. Mom would never let me near a carpet or couch with it. Basically, whenever I wanted to play Slime (and I use the word "play" loosely) I had to stand in the hallway or kitchen, unmoving but for the relentless twitching like a electric chair customer when trying to get the crap off my hands.
They actually came out with a sequel toy called Slime with Worms. I think they got the idea from the first kid who swallowed some of the stuff and ended up in the hospital with the creatures in his stool. Slime with Worms was purple instead of green and came in an orange trash can with crimson-colored rubber worms suitable for scaring a sibling or going fishing.
At least I could take the rubber worms in for Show and Tell without having them in the plastic can. It's a wonder I never did well in Show and Tell, and why, to this day, I can't stand eating wings or ribs because of the crap on my hands while eating them.
I hate you Mattel.