Is Marx considered an important classical economist?
Among economic historians, Marx is often considered the most important economist of the 19th century.  His attempts to provide a systematic explanation of the functioning of capitalism  was on a far grander scale than anyone who preceded him, and in that sense, he reset the bar very high for what would henceforth be considered comprehensive economic theory, even if parts of it were fundamentally flawed. Refer to Mark Blaug's classic text Economic Theory in Retrospect (Chapter 7) for a nice attempt to parse out Marx's economic theory from the rest of his ideas.  This is a standard and authoritative text in the history of economic theory. 

Blaug says of Marx "Marx the economist is alive and relevant today in a way none of the writers we have thus far considered." Those considered were Smith, Malthus, Say, Ricardo, &amp; Mill.  

In terms of his understanding of the dynamic long run evolution of economic systems, the nature of technical change, business cycles, and volume of employment, his insights were often brilliant, transformative and unparalleled in any writers that preceded him, and remain deeply influential.  (I'm paraphrasing Blaug here)

[It's worth mentioning that Blaug did his Ph.D at Columbian under George Stigler, who was a key leader in the Chicago School. So Blaug was hardly a lightweight, nor a "pinko" ;) ]