who was the senator who adopted a general as a son and began the era of the five good emperors
Trajan
Trajan
. In the Renaissance, Machiavelli, speaking on the advantages of adoptive succession over heredity, mentioned the five successive good emperors "from Nerva to Marcus"a trope out of which the 18th-century historian Edward Gibbon popularized the notion of the Five Good Emperors, of whom Trajan was the second.

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Trajan [SEP] Sources
Besides this, Pliny the Younger's "Panegyricus" and Dio of Prusa's orations are the best surviving contemporary sources. Both are adulatory perorations, typical of the High Imperial period, that describe an idealized monarch and an equally idealized view of Trajan's rule, and concern themselves more with ideology than with actual fact. The tenth volume of Pliny's letters contains his correspondence with Trajan, which deals with various aspects of imperial Roman government, but this correspondence is neither intimate nor candid: it is an exchange of official mail, in which Pliny's stance borders on the servile. It is certain that much of the text of the letters that appear in this collection over Trajan's signature was written and/or edited by Trajan's Imperial secretary, his "ab epistulis".