Most difficult to get through for me...
Hegel - Phenomenology of Spirit
Plotinus - Enneads
Kant - Critique of Pure Reason (just boring)
Voula Tsouna -The Epistemology of the Cyrenaic School (dry as saw dust, but I got through it somehow after several attempts)
Nietzsche - The Birth of Tragedy (I think I finished it, but very boring -- sorry orthonorm!)
Heidegger - Being and Time (I quit on this one fairly early, partly from boredom and partly from laziness I guess)