Most sociology undergrads will you tell you its Marx, or Weber, or Durkheim. In addition to citing those three, I'd bet a lot of graduate students and professors would name the major influences behind their particular theoretical approach or specialty.
Heres on you might not have heard before: Otis Dudley Duncan. Basically the introductor of modern regression and statistical techniques to sociology, grounding it firmly in empirical data and strict quantitative methodologies. Duncan's legacy is whats moved sociology (a great deal of it, at least) out of the humanities and into the true social sciences.
Before Duncan, we had a lot of literature that was insightful, yes, and well written, yes, but strictly scientific in most ways, it was not. An odd blend of economics, psychology, intuition and historical analysis, sociology pre-Duncan is markedly antiquated in comparison to the types of analyses and data sets common to modern sociology. This only gets more true as time passes.