Most fascinating but lesser-known Roman emperor?

[Antonio the Sixth]

Antoninus Pius has long been a favorite of mine. He avoided the sweeping military campaigns and wild internal projects of his predecessors and focused on sound and efficient administration, stamping out corruption, and maintaining good relationships with Rome's neighbors. His reign was largely uneventful, which is why he's not made a huge mark on historical memory, but for the people living under his reign that was probably a blessing. Although iirc he enacted the first regulations on the treatment of slaves, which while a low low bar was still a step in the right direction.