As a college junior, I studied the life of Spinoza. I carried books about Spinoza around for so long that a friend nicknamed me Spinoza--a moniker that I am still known by (at least by some old friends) today. So, it's an easy choice for me: I would love to have spent a day discussing life with Spinoza.

I would be interested in meeting Descartes. He says "I think, therefore I am," but I am always wondering why I think something. I am curious what he would have to say about the power of individual mind in opposition or collaboration with other minds. For example, "I think the house is yellow, therefore it is." But what is yellow? How do we know? Do we only "know" because humanity has agreed since the some unknown past time that yellow is that light bright color like the sun appears in the sky? Why is yellow even the word we associate with that color?

Ultimately, I am interested the power of thinking and how it shapes our understanding of the world around us. 

I'd like to meet Locke.  The reason that I would pick him is simply that he has had a lot more of an influence on my life than either of the other two.  His ideas about the consent of the governed and inherent human rights are the basis for our system.

I disagree.  I think you might want to meet one in order to question or argue with their beliefs, not necessarily because you agree.  For example, I might ask Descartes to support his argument about reason in the modern world, where reason is used as an excuse to commit atrocities.

While the question is phrased in the form of a choice, the essence of the inquiry implies an opposition to one or the other’s point of view.  For example, Rene Descartes, who championed Reason as the singular reliable epistemology (way of knowing) would be interesting for a pragmatist to meet, because they could agree on the basis of their mutual philosophies.  But more interesting would be a discussion between Descartes and a Spiritualist, or a Transcendentalist.  Such a conversation would elicit inquiries into the existence of things that did not submit to Reason, or physics, or scientific proof—mother love, fear, hope, a belief in an afterlife, etc.  The first thing I would say to him is “I think I think; therefore I think I am.”  This little statement punches a hole in his logic, and it would be interesting to hear his response.  Another line of inquiry might be, “I am--OK, but why am I?”  This might be called the existential rejoinder.  Descartes may have opened an interesting line of inquiry, but later philosopher have gone beyond his thought.  It would be fun to bring him up to date on Phenomenology, Speech Theory a la Wittgenstein, and Existentialism.