It really depends on what you want to know. All philosophy is written in conversation with what came before. If you want to understand the Western philosophical tradition fully, you pretty much need to start with Plato and work your way forward. If you want to understand Zizek, you probably need some Hegel, some Marx, some Lacan, and so on.

Is there a particular area of philosophy you're interested in? Phenomenology? Continental theory? That would make it much easier to make recommendations.