It's a pretty hard question to answer; I, for example, find the work of the German phenomenologists (particularly Husserl, and Kant if you place him in that category) to be absolutely fascinating, but their work could never have come about had it not been for Descartes, most of whose work I believe to be pretty abysmal.
Descartes revolutionized philosophy through the idea of Cartesian doubt and the veil of perception, and made phenomenology and modern epistemology possible, but his own resolution to that "problem" is deeply flawed.