This is a really hard question without knowing what your area of interest is. However, I find Descartes Meditations gripping for the way he poses the question

This is a really hard question without knowing what your area of interest is. However, I find Descartes Meditations gripping for the way he poses the question of whether there can be any solid foundation for knowledge and the resulting skeptical worries (though his solution is somewhat disappointing). Hume's Enquiry Concerning Human Understanding is also wonderful. If you find free will interesting, Kane's "A Contemporary Introduction to Free Will" is solid--it was the text a professor assigned when I ta'd for his course.