How to tell bad philosophers from good ones?
How to determine the "value" of a philosopher and his work?
How can we tell that e.g. Plato, Descartes, Kant or Marx were great philosophers while many around them weren't so great?
I'll start with analogy from different field. When we look back at history of science, we (at least in a simplified view) can say that the "good" scientists were those whose predictions about the nature of the world matched the objective reality. In science, what is true, is valuable, and vice versa. Some other criteria could be though of as well. One could say that Newton's and Einstein's theories were regarded valuable because they matched with objective reality AND explained things that weren't explained before AND could be used to build other theories and reasoning on top of them.
Now, what about philosophy? One could say that a good philosopher is a philosopher whose argumentation is good, i.e. convincing. But shouldn't in this case many lawyers be regarded as great philosophers? They also often have good argumentation skills. I see a possible objection that lawyers are not concerned with abstract and "eternal" questions, the questions of philosophy, and therefore "do not qualify" to be philosophers. But then, if that true, there exists a method to create many first-class philosophers. We just take good lawyers and order (or pay) them to do philosophy. Why this wouldn't work?