Tyler Cowen asks "Who is the worst philosopher?" The comments section is a remarkably amusing pile of utter ignorance in a state of pompous pontification. Cowen himself restricts it to philosophers of renown, which is a somewhat vague set, and concludes Husserl. I'm not a huge fan of Husserl, but this is, frankly, an absurd answer, even granted that the question is something of an absurd question. We don't even really get an explanation why, or any actual analysis of Husserl in comparison with other philosophers; in the comments, Cowen dismisses without much explanation the obvious reason to refuse to label Husserl 'worst', namely, his extensive influence on people who are obviously themselves not the worst, and the esteem in which they held him. Cowen suggests Plato or Hume as the greatest philosopher, and Aristotle and Nietzsche as the most overrated philosophers. I say this as very much a fan of Hume, but, if we were going this direction, it would be extraordinarily easy to argue that Hume is overrated: his current popularity is demonstrably Zeitgeist-driven and for the wrong reasons, having very little to do with Hume's actual philosophical activity, since it is very easy to find people attributing positions to Hume that are far less limited than he actually asserts. Plato is the obvious candidate for greatest philosopher, but it is an uphill battle to argue that Aristotle is overrated; the Prior Analytics and Nicomachean Ethics alone would still give him a serious claim to being one of the greatest philosophers in history on the basis of (1) substantive influence, (2) important argument and discovery, and (3) fruitful ideas. But saying that Aristotle is overrated as a philosopher is almost certainly like saying that Shakespeare is overrated as a playwright, or that Dickens is overrated as a novelist; we are dealing with a level for which 'overrated' could only possibly have an idiosyncratic meaning.
In reality, of course, it's a mistake to rank philosophers on a single scale; as Karl Jaspers pointed out, philosophers do very different things that are not all commensurable. Jaspers himself divided the field of philosophical greatness into six areas*:
(1) paradigmatic individuals, who provide by their practice extraordinarily influential models for philosophizing, namely, Socrates, Jesus, Buddha, and Confucius;
(2) seminal founders, who establish new large-scale traditions, namely, Plato, Augustine, or Kant;
(3) intellectual visionaries, who are the great philosophers whose excellence resides in a sort of conceptualization, like Spinoza or Leibniz;
(4) great disturbers, who are the great philosophers who excel at criticism, like Hume or Nietzsche;
(5) creative orderers, who are the great philosophers who excel at system-building, like Aristotle, Aquinas, or Hegel.
(6) specialists who excel at a particular area (e.g., aesthetics, or philosophy of law).
But it is clearly impossible to grade all of these different functions according to a single criterion. Critical problematics is a very different kind of activity from conceptual construction, for instance, with different ends, means, patterns of influence, and so forth, so greatness along one axis won't necessarily be relevant to greatness along the other, and weakness along one axis won't necessarily translate into weakness along another. People will of course end up answering such questions based on a mix of their (necessarily limited and flawed) understanding of the two and a half millenia history of philosophy and their own subjective taste.
If we're talking about taste, though, taste is only as good as its cultivation, which requires extensive experience, discriminative capacity and the ability to express it, and careful comparison. And that means only us historians of philosophy are competent to answer questions like this. The answer, of course, is that all major philosophers are underrated, taken absolutely, and you can't talk about about worst philosophers without asking specifically "worst at what philosophical activity", since there are provably many, and that Plato and Aristotle are the primary sources of philosophy in the West, and the Western philosophers who have been regarded as the most important philosophers by the most people who are serious candidates for being considered great philosophers, and such people are obviously the people whose opinion forms the standard of good taste in philosophy.
* Jaspers's scheme is, of course, only one of several possible. It's perhaps worth pointing out, however, that it is not arbitrary, since this criticism occasionally comes up. Paradigmatic individuals, great thinkers (2 through 5), and specialists have very different roles in the history of philosophy, and this can be traced fairly objectively by recognizing that the patterns of influence for each of these groups is very different. Among great thinkers, we have the vital activity of philosophy as conceptualization (3), analysis (4), and synthesis (5), and then unusual cases that combine these in ways that make it possible to build complex traditions on the basis of them (2). The problem with the scheme, if one wants to criticize, is not that it is arbitrary, or even wrong, since there is a great deal to be said in its favor, but that it is a scheme: if you take a philosopher like Berkeley, for instance, there is no way to capture both his chessmaster-like critical argumentation and his innovative theory of vision. Or, if we take Hume, how does emphasizing criticism really do justice to one of Hume's very great strengths, psychological observation, or the fact that Hume is quite deliberately system-building? Setting Aristotle down as a creative orderer simply doesn't do justice to his critical acumen -- Aristotle's entire systematic ordering is based on a thorough critique of previous philosophers -- or the extent to which Aristotle was inventing entirely new vocabularies for talking about entirely new fields of thought. In reality, of course, all philosophers do something of (3), (4), (5), and (6), so we must be dealing with proportions rather than set classes. And that, in fact, is certainly right.