Answer: Formulating Periodic Law
Dmitri Mendeleev was a brilliant Russian chemist, active in the mid-to-late 19th century, whose greatest contribution to the field of chemistry was the formulation of Periodic Law and the creation of the first predictive periodic table of elements.
While other scientists were wrestling with the idea of chemical periodicity (that chemical elements could be grouped by some intrinsic atom-level qualities) and making gains in the process, Mendeleev had a breakthrough that changed the entire discipline. In 1869, he presented his ideas to the Russian Chemical Society outlining key (and now familiar) ideas like: elements arranged according to their mass exhibit periodicity of properties, elements with similar chemical properties have similar atomic weights, and, most importantly, his theories, and the table he created using them, allowed not only for correction of existing errors in chemistry, but the prediction of then-unknown elements like aluminum and silicon.
Although his early version of the periodic table was imperfect—it could not be used to predict noble gases and there was no concrete way to place hydrogen, which could be placed in more than one location on the table—it significantly advanced the field of chemistry and our modern periodic table can trace its roots back to Mendeleev’s breakthroughs.