"The Open Syllabus Project (OSP) collects and analyzes millions of syllabi to support educational research and novel teaching and learning applications."
The most frequently assigned title across "6,059,459 syllabi" is Strunk's Elements of Style. The most frequently assigned title in economics is Mankiw's principles text. (Mankiw's books are also #2 and #3 in economics.)
For what economists should do about Marx's high ranking in economics, see this interesting post by Professor David Tufte.