Medium.

I like a slow, historically accurate game, and I don't want the AI attacking my faction for no good reason other than because its human-controlled.

Diplomacy actually works well on Medium. With Hard and Very Hard you need to use the Force Diplomacy mod, becuase the AI factions hate you more and won't make deals with you.

Also, Medium makes naval battles a lot easier, you don't have to outnumber your enemy three to one to win a naval battle.

Medium also helps a lot when playing the smaller factions like Hayasdan or Bactria, being able to make peace with their steppe enemies prevents you from being swamped on two fronts.

I would only pick Hard difficulty if I was playing a faction that was historically both very powerful, and universally hated by almost all its neighbours. Like the Ptolemies and Seleucids for instance. Maybe the Carthaginians and Romans as well.