Hi!
The highest rank is always the one from which you can not promote any further 
Now to be serious: one of the cool things about DeI is they overhauled this aspect of politics, meaning ranks and its boni differ from culture to culture! Since I mostly play Greeks I can say that Hetairos is the highest for them!
But other cultures have different ranks with different boni so there is no clear answer to your question.
On a sidenote: your Mr. Maximus seems to be a decent guy! Especially this vir triumphalis is a nice trait!
In my current campaign Hiero II enjoys the pleasures of life a little too much and bad traits come with it, resulting in this:
Really cool feature, as I always thought faction leader boni/mali only depend on ranks