Of course, this is a selection that is very much determined by my personal taste and my own sympathies.
(Image source: Hugo Rheingold on commons.wikimedia.org)
1. Karl Raimund Popper
A fellow Austrian and eloquent critic of totalitarianism. With critical rationalism, he presents a useful recipe for dealing with knowledge. He was also a good stylist and a mentor who wanted to teach interested people something in simple language.
Genius of geniuses. I have to admit that I only know most of his works from secondary literature (apart from the Nicomachean Ethics). He has formalized thinking to such an extent that one wonders how this was possible thousands of years before the invention of the computer.
3. Karl Marx
Disclaimer: I am NOT a Marxist. :p
However, Marx was a brilliant stylist and probably a very likeable person. His friend Friedrich Engels called him "Mohr" – which loosely translates to "Blacky". He would have resisted his successors, who were responsible for one of the greatest heaps of corpses in history. However, we learn from Marx to think processes through from beginning to end.
4. Ayn Rand
Finally a woman! One that opposes state encroachment, collectivism, and ill-founded morals. She tried, like Nietzsche in a different way, to introduce rationality into ethics.
5. Friedrich Nietzsche
It speaks for Nietzsche that I can read his works for pleasure. His agile style is unmatched. Above all, I have benefited greatly from his writing on the genealogy of morality. He was a highly intelligent man who easily saw through the concepts of his difficult predecessors, Hegel and Schopenhauer.
6. David Hume
This Scottish philosopher is also easy to read. He thought wisely about the conditions of our knowledge and our judgments.
7. Lucius Aennaeus Seneca
Especially in our difficult times, the Stoics are experiencing a renaissance. Seneca exhorts us not to waste our time and to use our resources wisely. The effectiveness of his thinking was shown by the fact that he was a wealthy and influential man.
8. Arthur Schopenhauer
The tremendous intelligence of this somewhat eccentric man makes reading his main work "The World as Will and Idea" an adventure. Schopenhauer builds on the works of the very dry philosopher Immanuel Kant on the conditions of our cognition. Unlike the latter, he had a sense of humor and was at odds with much of his fellow human beings. He also penned an easily read treatise on rhetoric and a collection of aphorisms.
9. Bertrand Russell
The very long-lived Englishman was certainly not a pop star, but a person with a lot of practical sense who knew how to express himself wisely on many contemporaries and topics of the time. I use his book on Western philosophy as a kind of travel guide and great overview of philosophical issues.
Plato can also be read simply for pleasure. His dialogues are like taking part in a discussion group, with the always somewhat precocious Socrates fooling his interlocutors with rhetorical tricks.
11. NOT these
I am either too superficial or unable to understand or properly appreciate the works of the following great thinkers: Incomprehensible brain acrobats like Hegel and Adorno. Deep talkers like Heidegger. Unctuous critics of Western societies like all members of the Frankfurt School. Good thing there are Wikipedia articles about these people.