I think mine have to be Hume and Wittgenstein. Hume, because of his work on ethics (namely the is/ought gap), the philosophy of science (problem of induction), early psychological work and his attacks on religion and apologetics arguments (I recently read his "On Natural Religion").
Wittgenstein is a newer one for me. I like his suggestion that the problems in philosophy are (at least oftentimes) most likely to be confusions of (caused by) language. It just seems to make sense to me. And his anti-Platonic work via his introduction of "family resemblance" concept is likewise convincing.
Quick question: Is his "Philosophical Investigations" a good read? I've been thinking about looking for it on Amazon lately.
What about y'all?
John Locke, Thomas Payne and believe it or not David Attenbourruogh!
Buddha and Sankaracharya. Buddha for questioning everything and Sankara for non-duality.