Becker gave econ a new identity

Gary Becker's work gave economics a new identity, and his influence is enormous in economics and beyond (see No. 19 here). Most of his academic articles were published in economics journals, and his work was initially ignored:

For a long time my type of work was either ignored or strongly disliked by most of the leading economists. I was considered way out and perhaps not really an economist. But younger economists were more sympathetic. They may disagree with my analysis, but accept the kind of problems, studied as perfectly legitimate. During the past ten years I have received much tangible evidence of this shift in professional opinion, including the presidency of the American Economic Association, the Seidman Award, and the first social science Award of Merit from the National Institute of Health.

By far, his most cited work is his book on human capital.  
A screenshot from Google Scholar citation-count:

He showed that rational choice is a powerful tool to understand life. In fact, he viewed rational choice as a unified approach across social sciences.