That was one of the questions I was asked at my Jane St. Capital talk on Wednesday night.
My answer was Edmund Husserl, at least if we restrict the question to philosophers of renown. I believe his work is a waste of time and I write that as someone who does not believe Heidegger is a (total) waste of time, especially in the essays. As for Husserl, we can pull this bit off Wikipedia:
Therein, Husserl in 1931 refers to “Transcendental Subjectivity” being “a new field of experience” opened as a result of practicing phenomenological reduction, and giving rise to an a priori science not empirically based but somewhat similar to mathematics. By such practice the individual becomes the “transcendental Ego”, although Husserl acknowledges the problem of solipsism. Later he emphasizes “the necessary stressing of the difference between transcendental and psychological subjectivity, the repeated declaration that transcendental phenomenology is not in any sense psychology… ” but rather (in contrast to naturalistic psychology) by the phenomenological reduction “the life of the soul is made intelligible in its most intimate and originally intuitional essence” and whereby “objects of the most varied grades right up to the level of the objective world are there for the Ego… .” Ibid. at 5-7, 11-12, 18.
I suggested both Aristotle and Nietzsche as overrated philosophers, although clearly both are still great philosophers, worthy of major reputations. But neither should be considered a real candidate for “greatest philosopher ever,” which is what you sometimes hear. I’ll reserve that for Plato and Hume.