Why were five Rome's rulers known as the "good emperors"?

Since it was from Alexander the Great and One of the Emperor was ruled by Herculus the god of War

The ancient Roman imperial succession of Nerva (reigned ad 96-98), Trajan (98-117), Hadrian (117-138), Antoninus Pius (138-161), and Marcus Aurelius (161-180), who presided over the most majestic days of the Roman Empire. It was not a bloodline; Nerva was raised to the principate by the assassins of Domitian, and the others were successively adopted heirs, each only distantly related to his predecessor if at all. The last two-Antoninus Pius and Marcus Aurelius-are often called the Antonines, though the term Antonine is sometimes extended also to the coemperors Lucius Verus (adopted heir of Antoninus Pius) and Commodus (son of Marcus Aurelius).