Reddit mentions of Leibniz: Philosophical Essays (Hackett Classics)

Which commentaries/reactions do you have? I, too, would be interested in getting commentaries on Kant, though mainly on CPR. He had some theological ideas (and stuff in a few disparate sections) I'd be interested on reading more about.

Can't say much about commentaries. However, I have a few ideas as far as who he's responding to goes:

I'd start with, as you listed Hume's Treatise on Human Nature. CPR is basically a direct response to that book (which is in turn a response to Plato and others) and at the very least he addresses those ideas first. Then I'd move on to an Enquiry Concerning the Principles of Morals. Hume considered it the best of his works, and from what I could tell the best expression of his ideas on them. Kant seemed to respond a little to the style in his own moral works, and it may prove interesting with the Canon of Pure Reason chapter and Kant's own moral works.

Descartes as well. His account of the cogito in his Meditations (and perhaps even Discourse on Method) would provide context for the Transcendental Deduction. Kant responds to several other ideas from his psychological works IIRC, though not explicitly. Kant critiques Platonic and classical empiricist ideas as well in CPR, so those may be worth looking into. He responds to the ideas of Newton at least once, though for the life of me I can't remember where.

There are several theologians that Kant is responding to as well, including Leibniz. However, there isn't just one "Leibniz book". The guy never wrote a complete expression of his philosophical and theological thought. There's a good edition out by Hackett that contains many relevant essays. Kant also seemed to respond to Aquinas' thoughts from The End of Man chapter in the Summa as well at one point in the Canon. Spinoza as well, though to be honest I haven't read his Ethics yet, though the Tractatus is my favorite book. However, as a result I can't confirm this, but I imagine they'd be in the Ethics.

As for responses, I'm less familiar in this area (yet!):

I've heard that Nietzsche responds to his ideas. But as I haven't had the pleasure of reading Nietzsche yet, I can't comment on that. However, I do know that Bergson responds to him briefly in Creative Evolution, which I am currently reading. Can't speak on that because I'm not there yet.

While I can understand reading Plato's and Aristotle's complete works, there's no point in doing the same for Cicero or Seneca. It's simply a waste of time. If you truly decided to go through with this plan, you'd be dead by the time you reached Plutarch.

If you're at all interested in modern philosophy, you need to get to Kant as quickly as possible. You don't need to have read the entire corpus of ancient and medieval philosophy to understand what's going on today. Also, primary texts alone won't cut it. Don't underestimate the complexity of these ancient texts simply because they're so old.

With that, here's what I would suggest:

1. The magnificent Leo Strauss recorded a bunch of lectures on different texts by Plato and Aristotle. Some of these recordings are incomplete but for Plato you can find the complete recordings of his lectures on The Republic, Gorgias, Protagoras, Laws, Thucydides and Meno. For Aristotle, you'll find his recordings on his Ethics, his Politics and his Rhetoric. Listen to them while you read these texts.
   
   
2. Put Seneca and Cicero aside for a while, they'll only slow you down right now. You can come back to them later.
   
   
3. You'll need some background to understand what Kant is doing. For that purpose, read Descartes mediations, Locke's Essay concerning human understanding and Hume's Enquiry concerning human understanding. The one philosopher missing in this list is Leibniz. It's not easy to point to one particular work of his, since he published mostly essays. Also, his thinking is much "wackier" and harder to get than the other one's here. With that in mind, get his collected essays and a secondary text on Leibniz. The routledge books are usually fine although I've heard very high praise for Bertrand Russel's A Critical Exposition of the Philosophy of Leibniz.
   
   
4. You'll now have the necessary background to get into Kant. I suggest you read at the very least his Critique of pure reason, Prolegemona, Grounding for the metaphysics of morals and critique of practical reason. All of these are tricky but absolutely crucial texts. WIthout them, nothing that comes after Kant will make much sense. For the CPR, get Gardner's Guidebook to the CPR. Also, here are two really great recorded courses on the CPR. The first is by J. Bernsetin and the second by Richard D. Winfield. Once you feel comfortable with Kant, go for the ultimate secondary text, Allison's Kant's Transcendental Idealism. For Kant's works on ethics, consider Allison's Commentary on Kant's Groundwork for the metaphysics of morals. Also, get Allen Wood's magnificent book Kantian Ethics.
   
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   You now have a solid foundation in the three fundamental thinkers of western philosophy. Now, all the doors are open. You could go further and either start reading Hegel and Adorno or alternatively you could just straight to Husserl and Heidegger. All these four thinkers require Kant as a basis but with Heidegger, the background reading in Aristotle will start to pay off. You could also go for Kierkegaard and Nietzsche if you're into existentialism.
   
   
   Alternatively, if you want to specialize in medieval thinking, refocus your studies on Aristotle, read Plotin and Augustine, get Edward Feser's books on Thomas Aquinas, learn Latin and get to work reading the Summa.
   
   One more thing: If you're really serious about reading basically the complete works of Plato, Aristotle and other greek thinkers, you'll need to learn Attic Greek. My favourite textbook is Mastronarde's, although if you want to go straight to reading texts, consider Reading Greek.