Einstein’s greatest legacy is not General Relativity, it’s not the photoelectric effect, and it’s not slices of his brain. It’s a word: Gedankenexperiment – that’s German for “thought experiment”.As the comments point out, thought experiments have a long history of being used by Galileo, Newton, and every other famous theoretical physicist.
Today, thought experiments are common in theoretical physics.
Einstein also liked to imagine how it would be to chase after photons, which was super-important for him to develop special relativity, and he spent a lot of time thinking about what it really means to measure time and distances.No, Einstein did not develop special relativity, and did not show that quantum mechanics must be wrong.
But the maybe most influential of his thought experiments was one that he came up with to illustrate that quantum mechanics must be wrong.
A thought experiment that still gives headaches to theoretical physicists today is the black hole information loss paradox.I don't think that this even qualifies as a thought experiment, as no part of it is testable.
So it is ridiculous to credit Einstein for the thought experiment.
But Einstein did popularize a style of thinking that has infected theoretical physics. It is the idea that physicists can do some abstract thinking about how the universe ought to be, write down some equations, and declare them to be physical laws. This thinking guided his own fruitless research into unified field theories, and modern work on strings. And work on multiverses and a lot of other ideas that are sillier than string theory.