Which Philosophers are most often cited as Materialists (aka physicalists)?
I haven't done any research on the number of citations, so instead I'll just mention some major historical touchpoints.
In terms of classical philosophy, Lucretius would be a well-attested example of a materialist; he owes much to Democritus, but less of his work survives. Stephen Greenblatt, the noted historian (and Shakespearean) has recently published a book on the importance of Lucretius entitled The Swerve: How the World Became Modern; I've not (yet) read it myself, but it has gotten excellent reviews.
In terms of current philosophers, much of the Analytic tradition is materialist: Davidson, Searle, Fodor, and Dennett come to mind-- but this is not to write off the Continental tradition, either: Sartre seems to be a materialist, at least in his Marxist works.
A bunch of the pre-Socratics can be thought of as materialist. Thales, for instance, thought that everything was ultimately water. There are a bunch of other philosophers that followed and argued that reality was fundamentally fire or air or whatever. Democritus thought the fundamental 'stuff' were small particles, which he called atoms. He is usually thought of as the father of materialism.
In the Modern Era, Locke is usually associated more with Epistemology and Empiricism, but he believed that reality was ultimately made up of 'corpuscules', which are basically atoms.
Contemporary philosophers generally don't refer to themselves as materialists, for a variety of reasons. The spirit of materialism has been preserved in Naturalism, the position that only the Natural exists, and is discoverable through scientific methods. The New York Times recently published a series of exchanges between Tim Williamson and Alex Rosenburg about Naturalism.
In order: http://opinionator.blogs.nytimes.com/2011/09/04/what-is-naturalism/ http://opinionator.blogs.nytimes.com/2011/09/17/why-i-am-a-naturalist/ http://opinionator.blogs.nytimes.com/2011/09/28/on-ducking-challenges-to-naturalism/