I figure the best is the 'merry go round'. Children, with unlimited energy, can play on these things for hours. All they do is go around in circles. Parental dream come true. I remember it from my childhood. Being on the machine was sort of like being on a ship, captain of the ship. Unfortunately, wet weather and muddy conditions enabled us to roll out huge 'turds' of mud. We would compare them for size. Children can ruin almost anything, but this is the best invention ever.
Worst invention, the mirror. I can only imagine what women might have been like before they could primp in front of the mirror. I am sure they would have cleaned their faces and done something with their nasty hair. Not spent hours primping. Back then they might have been cooking or cleaning or some such. Fetch me a beer or something.