Lists of great philosophers are usually rankings, which is a strange thing to do to people who disagreed about what greatness is. This is a different list: the ones a general reader gets something out of, with the specific book to start on, because starting with the wrong book is how most people conclude that philosophy is not for them. It usually is. They just opened Being and Nothingness.
The Greeks
Socrates wrote nothing and changed everything, which makes him the strangest entry on any list. Start with Plato's Apology: the trial, in his own defence, about 30 pages. Plato follows, and the mistake is opening with the Republic — start with the trial-and-death dialogues instead. Aristotle comes third and reads like lecture notes because that is what survives; the Nicomachean Ethics is the one.
Before all of them, Heraclitus and the rest of the Presocratics survive only in fragments, which turns out to suit them. After them, Diogenes lived in a jar and made an argument out of it, and Epicurus has been misquoted for 2,300 years by people who think he recommended luxury.
The Romans, who wrote to be used
Marcus Aurelius is the best first philosopher for most people, because Meditations was written for nobody and therefore argues with itself rather than performing. Epictetus was born enslaved and is the harder, clearer voice; the Enchiridion takes an evening. The Stoicism shelf puts them in order with Seneca.
The Chinese tradition
Laozi may not have existed; the Tao Te Ching exists regardless, 81 short chapters, and it is the one book on this page you read a page at a time rather than straight through. Zhuangzi is the funnier and stranger Taoist, and the better entry point if the Tao Te Ching feels like fog. Confucius is the third corner, and the contrast between them is most of classical Chinese thought.
The nineteenth century, where it gets personal
Schopenhauer is the most readable of the great Germans and much funnier than advertised; start with the essays, not the system. Kierkegaard invented most of what became existentialism while writing under pseudonyms who disagree with each other. Nietzsche is the most misread philosopher in the language; start with Beyond Good and Evil, leave Zarathustra until last, and know that The Will to Power is not a book he wrote.
In America, Emerson and Thoreau are stranger and better than the poster quotations made from them.
The twentieth century
Bertrand Russell writes the clearest prose in modern philosophy and is the safest recommendation on this page. Hannah Arendt is worth reading for what "the banality of evil" actually claimed, which is not what it is used to mean. Simone Weil died at 34 having published almost nothing and left the single most useful idea on this list: attention.
The existentialists: Sartre via the 70-page lecture rather than the 800-page book, Camus via The Stranger then The Myth of Sisyphus, and Simone de Beauvoir via the short one first, checking which translation of The Second Sex you are buying. The movement explained puts them in order, and absurdism is the branch Camus refused to call existentialism.
Later still, Foucault on power, which is not what most people think he means.
Beyond the standard syllabus
The canon above is narrow by construction, and the gaps are worth naming. B. R. Ambedkar wrote the Indian constitution and the sharpest modern critique of caste. Frantz Fanon is the essential reader on colonialism and psychiatry. Whole traditions sit outside the Western sequence entirely: African philosophy, Islamic philosophy, and Japanese philosophy.
And on the border between philosophy and practice: Alan Watts, who explained Eastern thought to the West brilliantly and with caveats, and Krishnamurti, who dissolved the organisation built around him and spent 57 years refusing followers.
If you only read three
Marcus Aurelius, for the voice. Plato's Apology, for what the whole thing is for. The Tao Te Ching, for the tradition that answers the same questions in a completely different key. That is about 250 pages and a genuine education.
If you would rather have the whole cast a page at a time, 365 Days of Philosophy (ours, flagged) runs 130-odd thinkers in roughly chronological order across a year, and the beginner shelf compares the good introductions, ours included and not flattered. For the questions they were all arguing about, see the great philosophical questions.
Common questions
Who are the greatest philosophers of all time?
By influence: Socrates, Plato, Aristotle, Confucius, Laozi, Kant, Nietzsche. By usefulness to a general reader starting today: Marcus Aurelius, Plato, Zhuangzi, Russell and Camus. The two lists differ, and the second one is the more useful answer to the question people are actually asking.
Which philosopher should I read first?
Marcus Aurelius. Meditations was a private notebook, so it argues with itself instead of performing, and it needs no background. Plato's Apology is the other good first door: 30 pages, a real trial, and the clearest statement of what the discipline is for.
Who is the father of philosophy?
Thales of Miletus by convention, for asking what the world is made of without appealing to gods. Socrates gets the title in popular use because he turned the subject towards how to live, which is what most people mean by philosophy.
What is the difference between Eastern and Western philosophy?
Broadly, emphasis. The Western tradition builds arguments toward conclusions; the Chinese and Indian traditions more often aim at a change in the reader. The contrast is real and routinely overdrawn — Zhuangzi argues, and the Stoics aimed at transformation. See Stoicism vs Taoism.
How many philosophers do I need to read?
Three, properly, beats thirty in summary. Depth in one thinker teaches you how philosophical reading works, and that skill transfers. Breadth without it produces a person who can name schools and use none of them.
Are there modern philosophers worth reading?
Yes, and the twentieth-century entries here are the accessible end: Russell, Arendt, Weil, Camus, Beauvoir, Fanon, Foucault. Contemporary academic philosophy is largely written for other academics, which is a reason to start with the essayists.