The Philosophy Bites podcast recently had an episode in which they asked various guests of theirs to name their favorite philosopher. I didn't do a tally, but it was clear that David Hume was far and away the most often mentioned, with Aristotle and Nietzsche following behind. Aristotle and Nietzsche are really not too much of a surprise; Aristotle was hugely prolific and ahead of his time, it's really difficult to grasp how such an achievement was possible. Nietzsche represented such a radical turn against received wisdom that philosophers almost have to admire, and coupled it with being such a complex and larger-than-life figure(for similar reasons, Wittgenstein was also a popular choice).
But were I asked the question, I'd probably throw in with Hume. His deep and thorough-going skepticism is really a work of art, and the fact that he contributed his greatest work by the age of 26 is also remarkable. That a young man could have such deep insight into the human experience sets a high bar for us all. Of course, I say this despite disagreeing with many of his views about morality (as far as moral philosophers go, I'd have to choose Kant.) But his critical take on what seemed obvious to many, and his fearless critique of religious dogma, seems to be truly unprecedented, and to some extent, unmatched since.
Socrates deserves honorable mention, though depending on my mood I might favor him over Hume. His personality and philosophical approach really are the foundation for modern ideas about critical thinking and method. To a certain extent, I think we should all aim to be more like Socrates, speaking truth to power, challenging people to be critical of themselves, thinking through all the implications of our views. Many found him annoying and threatening, so much so that they put him to death--but we should all be willing to do as much as he did for pursuit of the right and good.
Of contemporary philosophers, I'd choose either Derek Parfit or Christine Korsgaard. The way they make such complex and subtle views readily accessible represents the direction I hope philosophy is heading.