I find it LAUGHABLE to try to claim that Rand was the greatest philosopher since Aristotle! What about Anselm, Aquinas, Spinoza, Hume, or Kant, to name a few. In addition, there are many others that have made solid contributions to the field of philosophy in the 20th Century, such as Wittgenstein, Moore, Heidegger, and Popper. These philosophers can be recognized as great, even if you don't agree with their conclusions.
Of course they may be great, and nothing says they weren't. Personally I do not regard Anselm a philosopher at all, and Aquinas, though Ayn Rand gave him credit for reintroducing Aristotelian philosophy leading the way to the enlightenment, I do not regard him as great, because everything he said that was right came from Aristotle, and every original he said, was wrong. Spinoza was a mystic and did great damage to the field of philosophy. Descarte, Berkley, Hume, and Kant were nearly successful in destroying philosophy for ever (and in academia have been totally successful). Of course if you measure greatness by how much harm someone does, than these were certainly great.
Wittgenstein exchanged the floating abstraction of a disconnected language for knowledge (but very intelligently). Heidegger is just another mystic with no positive contribution to phiolosophy. Moore and Popper raise lots of interesting quesions--not exactly the purpose of philosophy.
You forgot some real greats by the way, which is revealing: Occam, Peter Abelard, Bacon, and, before Rand, the greatest since Aristotle, Locke.
Just my opinion.
Hank