Trajan. Under Trajan, the Roman Empire was at its largest size ever.
Hadrian - Hadrian built Hadrian's Wall in Britain, to prevent the Picts from invading Roman Britain. Hadrian died in 138 AD. From 138 AD until 284 AD, there were 32 more emperors.
Theodosius I in 395 AD. He split the Roman Empire into two parts. The western part lasted until 476 AD, when it became ruled by the Germanic people. The eastern part lasted until 1453, when it became part of the Ottoman/Turkish Empire.
↑Jesús David CHARRY-SÁNCHEZ, Alberto VELEZ-VAN-MEERBEKE, and Leonardo PALACIOS-SÁNCHEZ "A BRIEF HISTORICAL OVERVIEW" Caligula: a neuropsychiatric explanation of his madness Arq Neuropsiquiatr. 2021 Apr; 79(4): 343–345.Published online 2021 May 8. doi: 10.1590/0004-282X-ANP-2020-0358