Ludwig Wittgenstein - mainly Philosophical Investigations, Blue and Brown Books, and On Certainty. It gradually changed how I view the entire world around me, and dissolved my interest in philosophy, as well as in many other "sciency" things that - I realized - I was only interested in because of implicit metaphysical interpretations that ultimately stemmed out of conceptual confusion.
I would also nominate Wittgenstein for similar reasons. To this day I still consider him the "deepest" philosopher I've read, paradoxically because he managed to stay on the surface of things and not get tricked by alluring (but ultimately nonsensical) word constructs. The subtlety of his thought is both mindblowing and very pleasing aesthetically — has anyone matched his talent for metaphor yet?
