So a lot of philosophical writing references a sort of general canon, at least as far as western thinking is concerned. Other traditions I'm sure have their own details but I can't speak intelligently to them. The field is enormous, and without more specific information regarding your goals it is difficult to pinpoint the classics in subfields that may interest you. That said, there are a few highlights I think you're bound to come across reference or allusion to in many, many other works that would benefit you to get some exposure to. I am strongly biased toward a western analytical philosophy tradition--you'll want to check into things far more broadly than I'm recommending to find your own path. My little list is not meant to be exhaustive and is coming from personal memory of "aha!" moments in my own life, but will perhaps serve as a useful beach head for your own investigations:
* Descartes' Meditations
* Locke Essay Concerning Human Understanding
* Hobbes Leviathan
* Kant Critique of Pure Reason
* Kant Prolegomena
* Kuhn Structure of Scientific Revolution
* Hegel Phenomenology of Spirit
* Foucault Discipline & Punish
* Sellars Epistemology and the Philosophy of Mind
* Sellars The Scientific Image of Man
* Quine Word & Object
* Davidson On the Very Idea of a Conceptual Scheme
* Nagel What is it like to be a bat?
* Searle Minds, Brains, and Programs
Stanford Encyclopedia of Philosophy [0] has a ton of great summary articles and bibliographies that could definitely keep you busy for a few decades or so. I've never been tremendously into ancient or non-western philosophy, which is a deficiency I aim to correct one day, but there are a ton of great essays there as well.