Einstein's Happiest Moment: The Equivalence PrincipleAs this article and Eikipedia explain, the equivalence principle goes back centuries. Einstein was very happy about using it in a 1907 paper, but it was not because gravity was a realization of curvature, as he did not even know what curvature was at the time.
Paul Worden, James Overduin
Einstein's happiest thought was his leap from the observation that a falling person feels no gravity to the realization that gravity might be equivalent to acceleration. It affects all bodies in the same way because it is a property of spacetime -- its curvature -- not a force propagating through spacetime (like electromagnetic or nuclear forces). When expressed in a way that is manifestly independent of the choice of coordinates, this idea became General Relativity. But the ground for what is now known as the "equivalence principle" was laid long before Einstein, affording a fascinating example of the growth of a scientific idea through the continuous interplay between theory and experiment.
It is my understanding that what Einstein was actually happy about was that he figured out a way to use the principle to use special realtivity to show gravitational time dilation.
Special relativity is often described as a theory about constant velocity, but back during the early days, say 1995-2010, it was widely understood to cover accelerating particles also. Poincare proposed a couple of relativistic gravity theories, but the geometry was not understood.
Einstgein figured out how to sidestep having a gravity theory, by saying that gravity was like non-gravitational acceleration. That was enough to figure out clocks in a gravitational field.
People often say that general relativity is needed for GPS navigation, but I don't think that is true. It only needs special relativity, and this trick of Einstein.
As far as I know, this idea of Einstein was his own, and not plagiarized from anyone else. Maybe that is why he was so happy about it.