
Some from the canon:
Plato: Defence of Socrates, Euthyphro, Crito (translated by David Gallop)The Modes of Scepticism by Julia Annas and Jonathan Barnes (a beautiful collection of texts from Pyrrhonian scepticism and analysis of their arguments; the best way I know to meet ancient scepticism)
Descartes: Meditations on First Philosophy (The Cambridge University Press edition is very good; it's probably worth buying their Selected Philosophical Writings for a good basic Descartes text)
David Hume: An Enquiry concerning Human Understanding and An Enquiry concerning the Principles of Morals (I recommend the Oxford Philosophical Texts editions)
Wittgenstein: Philosophical Investigations (terribly, terribly difficult, but very rewarding)
A few contemporary things:
Gilbert Ryle: The Concept of MindWillard Van Orman Quine: From a Logical Point of View
J. L. Mackie: Ethics: Inventing Right and Wrong
Thomas Nagel: Mortal Questions (short essays, well-written, very accessible)
Bernard Williams: Ethics and the Limits of Philosophy
Michael Smith: The Moral Problem
Specialist, technical, odd:
The Frege Reader by Michael Beaney (for modern logic and the beginnings of contemporary philosophy in the analytic style)J. L. Mackie: The Cement of the Universe: A Study of Causation
William Kingdon Clifford: The Ethics of Belief (he argues, oddly but powerfully, that it's immoral to believe things on insufficient evidence or irrationally - an offense to morality, not just reason)
Since you bring up the question of free speech, however, let's consider that angle. Does the right to free speech give you the right to shout "fire!" in a crowded movie theatre? Or to perjure yourself? No, and presumably the reason is because such speech reflects a malicious intent to cause harm to others. I don't suppose for an instant that Oprah has such a malicious intent, but I think most HN readers would agree that by giving celebrities and quacks a platform from which to spread their stupidity, she does cause harm to others.
In The Ethics of Belief, W. K. Clifford said "It is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence." If you accept Clifford's reasoning (as I do), then I think we must agree that Oprah is wrong to believe as she does, and further compounds that error by spreading her beliefs widely. While it is not illegal to be wrong, I don't think it would be a huge stretch to suggest that persons spreading beliefs that they have no right to believe (per Clifford) are culpable at some level for the resulting harm. As far as I know, the fact that the speaker honestly believes what they are saying is no defense in cases of libel, slander or hate speech.
My view is that Oprah has a duty to question her beliefs, and seek out evidence that refutes them. If she chooses not to do so, she is still entitled to hold those beliefs privately, but not to encourage them in others.