Marcus Vipsanius Agrippa comes to mind. He was a friend of Augustus since they were young men, and was a key player in Augustus' rise to power and early administration. Agrippa was the one in charge during many of Augustus' victories, such as putting down Sextus Pompeius, alot of the Civil War victories (He was a key player in helping Augustus raise and lead troops for the war), going into Germania, and quelling an Aquitaine rebellion. When the war was over, he was the architect of many public buildings in Rome, helping Augustus raise public favor, as well as managing provinces.

Tiberius, Augustus' son, was also a co-emperor later in life. He was granted pretty much equal power around 13 AD, and was the heir of Augustus. However, this was a VERY short arrangement, as by 14 AD Augustus passed away. Tiberius was, before then, ignored so he cant really be counted but I felt it necessary to mention him.

Agrippa, Livia, and Augustus' other groomed heirs cannot be discounted, but our texts are more or less unanimous in agreeing that the most important confidante of Augustus' was Maecenas. Although not quite as old an acquaintance as Agrippa, who had accompanied Octavian from Apollonia to Brundisium after Caesar's murder, Maecenas was among the earliest of Octavian's supporters (he fought at Philippi and arranged Octavian's marriage with Scribonia) and by far the most politically adept. While it was Agrippa that won the war against Sextus Pompey and the battle of Actium, it was Maecenas who forged the Pact of Brundisium and was able to renew the Triumvirate at Tarentum. He was a brilliant politician, despite holding no official magistracies at all during his career--he represents the first of a growing class of imperial officials, the equites who were used during the Principate to fulfill imperial duties not to be trusted to senatorial magistrates and promagistrates. Some older (and particularly British) textbooks will call Maecenas Augustus' "Prime Minister"--while not really an accurate label, it gives some sense of Maecenas' importance. Maecenas was Octavian's envoy to Antony in 38, and during Octavian's absences from Rome during the civil wars governed Italy in his stead, and did a fantastic bloody job of it--Octavian granted Maecenas, in an unprecedented move, imperium without any formal magistracy or promagistracy. The social and cultural programs of the Augustan age, with their grandiose vision of Rome and attempts to really foster a genuinely Latin literary tradition owe almost everything to Maecenas--Maecenas was the literary patron among patrons, supporting such giants as Horace and Virgil (and Propertius as well, who alludes to, but does not openly name him), as well as lost or fragmentary authors like Rufus, Tucca, and Marsus, and ultimately much of what we associate with the Augustan age must come back to Maecenas' handiwork, even that which was the vague brainchild of Augustus