A. Cameron asks why I equivocate on Spinoza but include Leibniz in my list of twenty-five. I studied Spinoza's Ethics in a semester-long course with the great Harry Austryn Wolfson that was one of the high points of my undergraduate education, so I was inclined to put him in. Wolfson taught him as, in effect, the last and greatest of the Medievals. His course connected Spinoza to the great debates in the Christian/Jewish/Muslim tradition. Leibniz, on the other hand, had a profound influence going forward on Kant and others. In addition, I think the Ethics is really impenetrable without a guide [such as Wolfson's own magnificent two-volume study].
However, my advice in all such cases of uncertainty is, Read it! This list is deliberately kept short and manageable in the perhaps vain hope that a serious graduate Philosophy student will actually attempt it.
By the way, I notice that I made a mistake in the numbering [see Aristotle]. The list is actually 26, not 25, titles long. I just reviewed it, and I think I had read maybe 24 or so of the titles [including a mess of the medieval stuff] by the time I got my degree, but I was only twenty-three then, so perhaps I may be excused. Also, I had taken four courses in Mathematical Logic plus Nelson Goodman's course on his first book, The Structure of Appearance.
I pass over in silence the suggestion that I comment on books I have never so much as looked at. I may be a blogging hacker, but I have some shreds of principles remaining!