Oh so many philosophers... I like the existentialists quite a bit, Nietzsche, Wittgenstein (though I have to admit I've only read a little of his work directly, but I like everything I have read and learned about through discussions) and Sartre.
I really enjoyed reading Martin Buber when we read a little of his stuff way back in my Freshman philosophy class, though we only read a little, and I only really remember some bits and pieces.
I also have a bit of a soft spot for Hegel, though his writing is so dense it's hard to make myself read much of it in the years since I got out of school.
I've also enjoyed reading about the bits of Heraclitus's works that still exist.
Someone else brought up political philosophy which of course opens up a whole other realm of discussion. John Stuart Mill has always really really impressed me, and John Rawls, especially his work "A Theory of Justice" is pretty amazing (even if you don't agree with him, he's crafted such an amazing theory that to be taken seriously these days you pretty much have to at least address Rawls' "Theory of Justice" before you can really start talking about your own ideas.). Finally, even though I don't really consider myself a conservative (really I consider myself a moderate with liberal tendencies) I have tremendous respect for Edmund Burke. I wish more people did, if more of the "conservatives" of today were classical conservatives in the tradition of Edmund Burke, I think this country would be much better off.