The completion of the general theory of relativity in late 1915 is considered Einstein’s greatest and most lasting achievement.Some say that, but my guess is that more say that the 1905 special relativity theory was the greatest.
Special relativity profoundly changes all of modern physics. No one can be a physicist today without knowing special relativity. The most basic variables of physics are space and time coordinates, and special relativity told us how they were related. Suddenly things made sense that never made sense before. It is hard to imagine a more essential shift in thinking.
On the other hand, general relativity has not been very consequential. Without it, XX century physics would not have been much different. Contrary to what you may read elsewhere, it had no effect on the GPS satellite system, as GPS only uses what was previously known about special relativity. It solved some theoretical puzzles, but it would have been eventually seen as the logical result of combining gravity with special relativity, even if Einstein never worked on it.
Constructing a relativistic gravity theory was mainly a mathematics problem, and it is well-known that Einstein relied on mathematicians for most of the crucial ideas. The main equation is that the Ricci tensor is zero, which is not too surprising once you figure out the relevance of the Ricci tensor. General relativity is a nice theory, but it is just not that physically important.
Among those who see special relativity as the big breakthrough, there is also a wide divergence of opinion on how to credit Einstein. Some say that that the whole theory begins and ends with Einstein's 1905 paper, as an individual stroke of genius. But everything in that paper was done much better in papers by others published earlier.
Sauer's paper discusses Einstein's notes, but they are mostly worthless nonsense.