Einstein was the first (and it really was a breakthrough) to realize the exact nature of spacetime. He reformulated the Lorentz transformation to show that the passage of time actually changes in different reference frames. (Edit: apparently the reformulation was done by Poincaré, but he still had the idea of aether.)

Also he got his Nobel prize for his work on the photoelectric effect. He wasn't the first to come up with the idea of photons, but his model was the first that could explain the effect. https://en.wikipedia.org/wiki/Photoelectric_effect#20th_cent...