Socrate
I know that I know nothing.
Aristophanes, The Clouds
A comedy in which Socrates is a ridiculous sophist who teaches how to "make the weaker argument the stronger". A caricature, staged twenty-four years before the trial; Plato (Apology, 18b-19c) says it fed prejudices against him. Not yet in our corpus.
Plato: Apology, Meno, Phaedo, Republic… essential
The richest and most famous Socrates. These are Plato's words, and Plato is no stenographer: in the early dialogues he is probably closer to the historical Socrates, in the later ones (Republic, Phaedo) he increasingly carries Plato's own ideas. See the Plato entry.
Xenophon: Memorabilia and Apology essential
A disciple defending his master's memory. Xenophon's Socrates: simpler, more moral, more practical than Plato's. Xenophon was not in Athens at the trial; his Apology rests on another man's report (Hermogenes).
Aristotle: remarks on Socrates (Metaphysics, Nicomachean Ethics)
Aristotle (born 384 BC) never knew Socrates. He reports, critically, what was ascribed to him in Plato's circle. A second-hand witness, but the coolest.
Diogenes Laertius, Lives, book II (Socrates)
A late compilation, six centuries after his death: anecdotes, quotations from lost sources, sometimes errors. Useful for what the tradition said, not as direct evidence.