An interesting bit of trivia is that Einstein explicitly did not get his Nobel for relativity. That was still considered too controversial at the time.

Another bit: at the time Einstein worked on Brownian motion the theory of atoms was still not entirely accepted by everyone. His worked helped settle the matter.

Actually I consider that by far the most valuable theoretical advance due to Einstein is the concept of stimulated emission, besides spontaneous emission and absorption, which has lead to the development of masers, lasers, atomic clocks and many other related devices, which have become an indispensable part of modern life.

This is also one of few of his works, together with the general relativity theory that he published around the same time during WWI, where he introduced new mathematical relationships between physical quantities, instead of just giving new interpretations to relationships already found by others.