Einstein did a lot, he not only published the special and general theories of relativity, he also contributed to quantum physics and was the first to deduce the size of an atom using statistical arguments, essentially proving the existence of atoms.
The size of an atom was computed for the first time 40 years earlier, in 1865, by Johann Josef Loschmidt, who was an Austrian scientist.
Loschmidt had determined the value of what today is called "Avogadro's number", despite the fact that Avogadro had no idea about how one could find the value at that number. The contribution of Avogadro had been the law that ideal gases have the same number of molecules per volume when the pressure and the temperature are the same, but he did not know how that number might be determined.
Based on the results of Loschmidt, which were also used by Maxwell in his works on statistical physics, a few years later, in 1874, George Johnstone Stoney computed the value of the elementary electric charge, which he later named "electron".
Loschmidt was the one who first measured and weighed the atoms, establishing their existence beyond any reasonable doubt. Before him, many believed that the hypothesis of Dalton about the existence of atoms is just a convenient theory for explaining the laws of chemistry, but some other better explanation might be discovered in the future.
Einstein published an explanation of the Brownian motion, which verified that everything fits in the known theories and it matches the expected values. It did not contain any novel number or law, but it was a useful confirmation of the current theories.
In my opinion, the greatest achievement of Einstein has been the paper published by him in which he revealed the existence of the stimulated emission of electromagnetic waves by hot bodies, besides their well known absorption and spontaneous emission.
This paper had tremendous practical applications, leading to the discovery of masers and lasers, without which the modern technology would have never existed.
While Einstein's theory of gravitation was more intellectually challenging, its practical applications are minor, almost negligible. Moreover, even after more than a century we do not know exactly how adequate it is for describing gravitation.
What is certain is that Einstein's theory of gravitation cannot be anything else but an approximation, as it is based on averaging the distribution of matter in space, without taking into account the concentration of matter into elementary particles.
Another contribution of Einstein that is more important than his most frequently cited works is his support for Satyendra Nath Bose, which lead to the recognition of the Bose-Einstein statistics, which together with the Fermi-Dirac statistics plays an essential role in quantum physics.
I don't understand all this dismissal of Einstein. What you just described is that he published a unified theory of Brownian motion, developed a new way of conceptualizing scientific theories via principles with which he derived special and general relativity, contributed foundational work to quantum mechanics, and more, but no, he was not one of humanities' greatest scientists.
What I have written is not a dismissal of Einstein. On the contrary, I have said that Einstein is the source of a few extremely important discoveries, but those are not the theories cited by most people, who have never read the original work of Einstein but speak about him from hearsay.
As I have written above, when speaking about Einstein, almost nobody mentions the discovery of stimulated emission, which has been far more important than anything else done by Einstein. Einstein did not discover special relativity, the photoelectric effect or Brownian motion. He just provided alternative explanations for them, which did not include any new formula or quantity.
On the other hand, stimulated emission was a surprise, even if it is one of those things that appears to be obvious in retrospect, but for decades nobody who studied the blackbody radiation had noticed that something is missing in the equations.
This discovery eventually lead to the invention of masers and lasers and today there exists almost nothing that incorporates modern technology and which could exist without lasers.
Already for decades, no modern integrated circuits, no CPU and no memory can be fabricated without using lasers. Even many purely mechanical things, but which require great precision, cannot be fabricated without using measurement instruments that depend on lasers.
So the contributions of Einstein are very important, but they are not those that most people associate with him. Besides stimulated emission his most original work was his theory of gravitation, a.k.a. general relativity, but if he had not developed it that might have changed nothing, because Hilbert had developed an equivalent theory almost simultaneously, so had Einstein not published first, Hilbert would have become its discoverer.
After stimulated emission, the most important in practice research of Einstein is the Bose-Einstein statistics. But again, here like for special relativity, the statistics had been discovered by someone else, i.e. by Bose. Einstein has nonetheless the great merit of understanding its value. He demonstrated that the statistics was much more generally applicable than initially believed by Bose and his authority convinced everybody about its importance.
Thus there is no doubt that Einstein is one of the most important physicists of the 20th century. Nevertheless, there are dozens who made similarly important discoveries, e.g. Sommerfeld, Dirac, de Broglie, Schroedinger, Fermi, Rutherford and many others.
