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)
Gosh another person like/similar to me! Thanks!