[theapportioner]

Philosophical Investigations by Wittgenstein. It rules. I'd try to get an idea of what logical positivism is about (Carnap, etc.) before you tackle it, though.

As far as the classics go, I like Aristotle's De Anima.

Maybe not "philosophy", but Saussure's Cours de linguistique generale is a canonical text and sets the stage for a lot of postmodern shite.

GE Moore's Principia Ethica.

Kuhn's The Structure of Scientific Revolutions.

Kierkegaard - Fear and Trembling