Ancient Sources

Julio-Claudian Emperors

 * Res Gestae - Augustus' description of his achievements as emperor. It is the only first-hand, first-person account of his reign. The civil wars that brought him to power are glossed over, and the Varian Disaster is not mentioned at all, and tries very hard to uphold the image that Augustus was the protector of the republic. May embellish some positive aspects of Augustus' reign. Well-preserved, as it was carved in stone and the actual stones were recovered.
 * Velleius - Contemporary of Augustus and Tiberius. He paints a very positive, sycophantic image of Augustus and Tiberius, and he is the only source to write positively about Tiberius - Tiberius was his patron. Velleius himself admits much of his work was written hastily. The text is badly preserved and corrupted.
 * Ovid - Roman poet and contemporary of Augustus.
 * Horace - Roman poet and contemporary of Augustus.
 * Virgil - Roman poet and contemporary of Augustus.
 * Cassius Dio - Senator of Greek origin writing in the 2nd and early 3rd centuries AD. Had access to the senatorial archives. Dio attempted to emulate Thucydides in his writing style. After the death of Claudius, his work is fragmented and much of it was reconstructed by later historians.
 * Seneca the Younger - Stoic philosopher and tutor to Nero. As a Senator he had access to senatorial archives. His writings were designed to try and teach Nero how to be a good ruler.
 * Strabo - Greek geographer and historian, generally writes factually and offers little in the way of opinion (much of what he writes about, i.e. geography, cannot really be opinionated). His work is well-preserved.
 * Suetonius - Roman equite. Did NOT have access to senatorial archives, but would have had access to many sources that used them, and various other records. His work, The Twelve Caesars, is written as a biography, written in roughly Hadrian's time. He is generally pro-republican and his work generally has a certain dramatic flair, often making references to superstition (omens etc).
 * Tacitus - Roman senator who had access to senatorial archives. Huge supporter of the republic, and generally very critical of the emperors, although he does recognise the positive contribution some of them made, and with the exception of Gaius does not wholly condemn or praise any emperor. He lived under the rule of Domitian, a corrupt and tyrannical ruler, which is likely what made him so negative about the emperors.

Roman Britain

 * Tacitus - Was the son-in-law of Agricola, so his history is geared towards painting Agricola in the best light possible, by emphasising his acheivements, glossing over his failings and possibly diminishing the role of other governors of Britannia to make Agricola seem better by comparison.
 * Strabo
 * Cassius Dio

Greco-Persian Wars and Peloponnesian War

 * Herodotus - Greek living in Persian-controlled Halicarnassus. His work includes a large amount of legends and fanciful accounts, but Herodotus states he merely reported what he was told by witnesses. Both Aristophanes and Thucydides criticised the less believable portions of Herodotus' writings. However, the more fantastical portions are obvious, and there is no reason to doubt the rest of his work.
 * Thucydides - Contemporary of the Peloponnesian War, fought for the Athenians, and was exiled after failing to defend Amphipolis. After his exile he observed the Peloponnesians, getting to understand both sides. As an Athenian patriot he is supportive of Athens, but is critical of Athenian politicians and respects the Peloponnesians as an enemy.
 * Xenophon - Athenian soldier and student of Socrates, contemporary of the Peloponnesian War (but only an adult in the later stages of it). He served under the Spartans and was pro-oligarchic, meaning despite being an Athenian he has some pro-Spartan bias. His work is intended to be a direct sequel to Thucydides, essentially picking up where he left off.
 * Aristophanes - Athenian satirical comic playwright. Harshly critical of both sides but especially the Athenians in the Peloponnesian War. His claim that the war was started over an argument over prostitutes is a mockery of Herodotus' claim that the Greco-Persian Wars were started by the rapes of mythical heroines, but his criticisms of the Megarian Decree are serious.

Philip

 * Justin - Latin Roman who (probably) lived in the second century, almost nothing is known about him. Generally critical of King Philip II's methods but does admit his successes.
 * Demosthenes - Athenian politican, the only contemporary of King Philip II. His speeches are trying to rally the Athenians against the growing power of Macedon and he is harshly critical of Philip. Many of his accusations have very flimsy evidence and are likely just slander.
 * Suetonius - The only positive source about King Philip II, and very positive at that, almost never criticising any of his actions.

Alexander

 * Plutarch - Greek biographer, whose work was intended to portray both famous Romans and Greeks as equally skilled and sharing common virtues and vices. More concerned with character than actual history, and many of the events he depicts have a dramatic flair. Generally positive about Alexander.
 * Arrian - Roman historian and general of Greek origin, knows a lot about the military so he offers detailed and realistic portrayals of Alexander's military exploits. Arrian is generally reliable but he is something of an apologist for Alexander, the main exception being Alexander's orientalism, of which Arrian is very critical.