Favorite crystal

Asymmetric unit of 1E9Z - crystal structure of Helicobacter pylori urease.

I nominate urease. It demonstrated that proteins actually HAD a structure, instead of being some amorphous glop...

From Wikipedia:

James B. Sumner

At Cornell, James B. Sumner began his research into isolating enzymes in pure form, a feat which had never been achieved before. The enzyme he worked with was urease. Sumner’s work was unsuccessful for many years and many of his colleagues were doubtful, believing that what he was trying to achieve was impossible, but in 1926 he demonstrated that urease could be isolated and crystallized (http://en.wikipedia.org/wiki/Crystal). He was also able to show by chemical tests that his pure urease was a protein. This was the first experimental proof that an enzyme is a protein, a controversial question at the time.

His successful research brought him to full professorship at Cornell in 1929. From 1924 on his laboratory was located on the second floor of the new dairy science building, Stocking Hall (today home to Food Science), at Cornell, where he did his Nobel prize-winning research. In 1937 he succeeded in isolating and crystallizing a second enzyme, catalase. By this time, John Howard Northrop of the Rockefeller Inst. had obtained other crystalline enzymes by similar methods, starting with pepsin in 1929. It had become clear that Sumner had devised a general crystallization method for enzymes, and also that all enzymes are proteins.