Which Roman Emperor led the Roman Empire to reach its maximum territorial extent?

The Roman emperor Trajan led the Roman Empire to reach its maximum territorial extent primarily through his military conquests and successful campaigns. Under his rule from 98 AD to 117 AD, Trajan expanded the empire to its greatest size through the annexation of Dacia (modern-day Romania) and the conquest of Mesopotamia, which included the capture of the Parthian capital of Ctesiphon and the entire region of Nabatea. Trajan's military victories significantly expanded the boundaries of the Roman Empire, allowing it to reach its largest territorial extent in history.