Different people have very different opinions and criteria about who is and isn't a great philosopher. Some say Ayn Rand is a great philosopher, I'd say she's just a crude creep who encourages other crude creeps to feel good about themselves, when they shouldn't. Recently I finally broke down and read a book by Karl Barth, Einfuehrung in die evangelische Theologie, because some people insisted that Barth was the greatest philosopher of the 20th century. After hearing such high praise, I was hugely disappointed. (Number of Christian theologians who have impressed me at all: still 1: Kierkegaard. And he only impresses me in those moments when he stops being theological.) But I might still actually dislike Barth less than Popper, whom I very recently read, submitting to the rave reviews of many different acquaintances. And I really don't know what to think of Sloterdijk, or Heidegger. They're both much more interesting than Rand, Barth or Popper. I don't know whether Sloterdijk and Heidegger are driving at things I approve or disapprove of. Either way, I approve very highly of interesting prose, and I'm not being flippant here, quality of prose is far from a trivial thing for me. I've finally figured out that I thoroughly loathe what Spengler stands for, and completely disagree with the basic tenets of his philosophy, which compared cultures to organisms -- but the man can write.
Hegel is one of the most highly-esteemed philosophers of the past 2 centuries. And yet, his contemporary Schopenhauer called him worse than worthless, a charlatan who heaped together piles of bullshit meant to sound like philosophical statements. I think Schopenhauer was exactly right about Hegel. I think Schopenhauer was right most of the time. Then again, he wrote some stuff which was utterly stupid, some of it antisemitic, a lot of it sexist.
I'm not sure how far you'd get as a philosophy student quoting Schopenhauer approvingly on the subject of Hegel. I suppose it would vary greatly from school to school, and would generally be risky to your grades. Much riskier still might be if you agree with Nietzsche on the subject of Plato. For many if not most people interested in Western philosophy, Plato is one its pillars and triumphs. For Nietzsche Plato is a catastrophe for Western civilization and a very, very bad man.
To answer the title of this blog post by not answering it: I would encourage you to decide for yourself who and what a great philosopher is, because a true philosopher truly thinks for him- or herself, and if you're not a philosopher yourself what's the point in studying philosophy? Ah, but I suppose that many of you asking who and what is a great philosopher are primarily philosophy students, on your way to becoming philosophy professors, and while I suppose it's possible to be one of those and also truly be a philosopher --
Don't put too much stock into my skepticism, or Schopenhauer's skepticism, about the conflict between being a philosopher and being a professor of philosophy. Schopenhauer's bete noir Hegel was a fabulously successful philosophy professor, and I failed in academia because I'm autistic and I was undiagnosed when I was a student.
That doesn't necessarily mean that Schopenhauer and I are wrong. Decide for yourself.
Then there's the question of how much my opinion of whether you're really a philosopher or not counts. Decide for yourself about that -- if you really want to be a philosopher. (It's not for everybody.)