For me, Schopenhauer, Bentham, Hume and Singer, hands down, all because of their clear consistent views on ethics, and their rational and / or empiricism.
There are of course others I like very much (Nitchsze stands out) , but the ones above are my favorites.
I really don't like Descartes. He's clearly more intelligent than I am, this is obvious from his pensées, but I can't see how he could justify some of his claims at all (some of them which were very cruel IMO)
For me Russell, Voltaire, and Socrates.
During a Feminist Theory class, about 12 years ago, I ran into what I though was a funny problem. The prof. rejected a paper concerning the statistical change over time of the income differences, women and men, given education, age, etc.
I was a philosophy major/math minor at the time.
The paper showed that income disparity seemed to be dropping and that the difference seemed to be dependent upon mostly age and education. With present population groups having the least disparity. The disparity seemed to grand-fathered in to the older populations.
The prof. rejected the paper on the basis that mathematics can not prove or be insightful on a philosophical question. I quess I came from more of a natural philosophy background, thinking that all our tools should be used to extract some measure of truth. Locking up 'philosophy' into a class separate from the sciences, which I thought was the promising child of philosophy, could denign it a tool, and condemn it to a state similar to theism.
She also, suggested that I read Kant! I thought I had, not my favorite!
Does this whole thing remind you of a turf war? Maybe I am just more fluid in my approach..;p)
In a sense I suppose it is a turf war, if philosophers are attempting to do the same thing that scientists do (understand reality) but with different means (speculation as opposed to evidence). So, wouldn't religion be part of the same turf war? When I grade history papers, I write "evidence?" next to things students write that I don't think they can support with facts; perhaps I should write "interesting philosophy" instead.