Pythagoras the basis of philosophy? No way. For one thing, nothing he wrote has survived, and the stories his followers told are clouded by myths and legends. In terms of actual ideas, Pythagoras may have done some interesting things in math, but Euclid (of Alexandria) did considerably more.
Many people agree with Whitehead that everything in European philosophy is a series of footnotes on Plato. That being said, some modern philosophers would protest that there are lots of footnotes.
One question is, "Why Plato?" Or more accurately, "Why Plato's teacher Socrates, Plato himself, and Plato's student Aristotle?"
My philosophy teacher answered, "Because they are the first ones that got it right."
Nietzsche was brilliant, but he was not a philosopher. Similarly with Freud. In fact almost all philosophers are brilliant, but that doesn't necessarily mean that they are correct. The question you want to ask yourself is how do you tell if something is worth reading? After all, you only have one lifetime, and there are several lifetimes of philosophical reading out there.
At it's heart, philosophy is a love of wisdom and truth. If it strays too far from that, either it gets disregarded or it leads to fatal ethical and other mistakes.
To give one example, Edmund Husserl founded phenomenology, and his two greatest students were Edith Stein and Martin Heidegger. Because of what they believed, Stein died a martyr and is now a Catholic saint, while Heidegger died an unrepentant Nazi sympathizer (and pretended that his philosophical beliefs had nothing to do with his political ones; go figure). So... will you read "Finite and Eternal Being" or "Being and Time"?