Nintendo made some crazy ass accessories over the years, but if we are looking at really expensive and useless accessories you kind of have to stand back in awe and really admire the completely impractical nature of R.O.B.
R.O.B. stood for Robotic Operating Buddy and was meant to be a simulation of a second player, and it looked like a robot so all of the world was super-stoked for the addition of such an accessory. The illusion was put out there that this contraption could actually function as some sort of competitor or a sidekick in gaming, if it was necessary. It turns out it was none of those things but was just a really expensive and simplistic remote controlled clamp.
When anyone says that Gyromite was the best game that was compatible with R.O.B. that really isn't saying a great deal. There were only ever 2 official games designed for this thing because it really couldn't do anything other than spin these disks that would sit on platforms and hold down buttons for a certain amount of time that would keep certain gates open inside the Gyromite game.
If you failed to get the disks on the disks on the button lever in time, the gate would either be closed thus not allowing our little fella in a lab coat to carry on in the direction he was going. Unlike almost every other game in the history of time, you didn't control the doctor, you controlled parts of the environment and you were meant to do this using the stupid robot buddy and his mechanical arms.
Of course the game became more complicated at the levels went on but the makers of the game must have known that players who got frustrated at the rather simplistic and stupid mechanics of your robotic friend could easily be bypassed by simply picking up the controller than the robot was connected to and pressing the buttons with your fingers!
As soon as a player did this the game became infinitely easier, the robot accessory was negated as useless, the Gyromite game was defeated with relative ease, and was likely never played again other than to poke fun at it.
R.O.B. was a terrible accessory and even Nintendo knew this because when it was tested with test audiences everyone was pretty unimpressed with it and little kids started to bypass him and just hit the buttons themselves rather than using the completely unnecessary middleman to play a game. They released him with as an optional pack-in for the top-priced NES base system, but only 1 other game was ever developed that used ROB. I owned ROB and eventually we loaded him up with firecrackers and sent him to the great plastic beyond.