Trajan was a Roman emperor (98 to 117 CE), the second of the Antonine dynasty. Not only is he counted among "the five good emperors " (from Nerva to Marcus Aurelius), but he was traditionally called "optimus princeps" (the best princeps/emperor). He extended the borders of the Roman empire to their maximum size, conquering new territories in Dacia and the Middle East.
Even today, the tourists in Rome have the opportunity to admire the remains from his reign - Trajan's Forum, Trajan's Market and Trajan's Column (with a statue of St. Peter at the top).
He didn't leave a son, so he adopted Hadrian, who succeeded him as emperor.