My vote goes to Vermillion City's Gym.

There was absolutely zero thought put into this, just a bunch of trash cans layed out in a grid with two buttons under them to disable a barrier blocking the Gym Leader. And if you don't get the second switch it resets the entire puzzle and moves the first switch to another random location. What the fuck were they thinking?

The worst part is that this Gym has appeared in FIVE FUCKING GAMES and they didn't change it one bit in any of them.