Homer and a few Greek plays
Sample a bit of the great story teller Herodotus, then read the birth of historiography in Thucydides' History of the Peloponnesian War, which by itself is also very interesting and important (wonder why the Founder of the US didn't like direct democracy? There are very important object lessons in it).
Surely Plato and Aristotle deserve some attention! The contents of the latter's Rhetoric is essential for when you can't reach people with dialectic.
Euclid's Elements is still about as good as you can get for what it teaches.
Plutarch is great, but I really like that period of history. To it I would add reading some of the earlier bits of Livy.
Read, or better yet listen to audio of a few of Geoffrey Chaucer's Canterbury Tales, out loud you can follow their Middle English.
Machiavelli's The Prince is still damned good, and a landmark in talking about politics as it is, not as how people would like it to be.
Shakespeare surely needs some attention by English speakers. Swift's Gulliver's Travels were amusing when I read them in their original, and obviously very influential.
So, yeah, check out some of the classics.