Emperor Hadrian.
Most people only know him as the guy who built Hadrian's wall. But he was so much more than that, he deserves to be considered the equal of Augustus and Julius Ceaser.
When roman Emperors seized the throne it's was almost always accompanied by at least one murder if not a civil war. But not Hadrian he seized the throne without spilling a drop of blood. He wasn't even related to the old emperor he just seized the throne through pure force of cunning whilst he was serving in Syria about 2000 miles from the capital city. The senate basically got three messages at once.
The emperor is dead long live the emperor
The eastern legions have declared support for Hadrian.
Hadrian is the new emperor all hail Hadrian Augustus.
Hadrian almost succeeded in his plan to unify and rebuild ancient Greece to it's former glory.
He stabilized the empire.
He can be largely credited with empire's longevity he's the one who decided to stop wars of conquest and turn the legions into a defensive force. Building many astounding feats of engineering to fortify the empire. Ironically probably the least impressive was the Britannic wall now known as "Hadrian's wall".
He led a successful campaign to purge corruption from the empire.
So if he was so great why isn't he more famous?
Because he was openly gay and Victorian scholars did not like that one bit. But his legacy was far too great to ignore so they decided to focus on his engineering and architecture and erase Hadrian the man from the history books. Also because he sadly died before he succeeded in restoring Greece.
Plus side note the first thing he did was to accidentally revolutionize Roman male fashion. He wasn't just openly gay he was the worlds first fashionista.