If all the land you rule over is incorporated as 1 state and you're a monarch then you hold one of several titles, King/Queen and Grand Prince/Princess all indicate hereditary lineage of ruling.
A Grand Duke or Archduke is a ruler that is a Military leader. This is not supposed to be a hereditary title, but often becomes one. Duke is also the title that a King's siblings fall under, because they are essentially peasants and are expected to do all the things that peasants do, such as go to war, but they also have connections which afford them armor and men to lead which naturally forces them down the path of a Military Leader.
A Caesar, Princeps, or "First Citizen" is a monarch for life by popular consent rather than primarily military or hereditary claims. That is the claim at least made by people who take this title, but are more often than not they are just another emperor or king under a different name.
Everything beneath this level are just lower divisions of management...
Above this level there is High King or King of Kings which is more along the lines of a head-of-the-family of the ruling family which has divided up their family holdings into kingdoms. They're not "a king" in the same way as a king is to a kingdom as the rulers over the sovereign state. Think of it more like what your dad can do - each of your siblings has their own car that in all rights is their car and as such he has no say over what you do with that car, but you will still likely listen to him and people will still go to him when you or your sibling messes up with the car. Now just change car for kingdoms, siblings for Kings, and Dad for High King and it's that idea, same extended down the family tree and passed in much the same way as kingship might be.
Another is President or Prime Minister, both of which are rulers of multi-sovereign states, but probably not what you're looking for and are more modern. The former isn't a ruler at all in theory and the title is derived from their job of presiding over a congress and is meant to not be prestigious at all. Prime Minister is exactly what the title says it is. A CEO would be the perfect comparison for what a Prime Minister is, they are the top level boss.
Emperor is the title for when you directly have the final word of multiple sovereign states without being related in such a way as a High King might be. The difference is mainly how those ruled recognize those who are ruling, either as foreigners or not, and whether they are incorporated into the body of the nation or not. Colonies and foreign territories are not incorporated into what is considered - the US for example which makes it not an empire, but if they did, while still maintaining they are separate sovereign nations, the US would become an Empire and the President would be an Emperor. Queen Elizabeth is an Empress, but because it is more or less just "technically" true, most ignore it.
Khan is on the same level as king, but Genghis Khan is combination of Emperor and High King due to the fact that everyone not of a given state, village, kingdom, was considered a foreigner to some degree, but they were all the same "culture".
Caliph is a religious absolute ruler, separate from the secular ruler, a Sultan, who rules over a religious "empire" which has Sultanates beneath it. This is pretty much the same things, but a more defined version of what happened during the middle ages with the Pope being the true absolute ruler from which all Kings derived their right to rule from.
Everything else is pretty much just different word for the same concept, and you should notice with "Caesar" that terms can derive from a person's name.
With regards to the term Governor. The title literally means "One who Governs" and is a perfectly adequate term for what you're looking for. A Governor in modern days governs a state in the US and has a Congressman, Senator, and President above them and Mayors beneath them, but the title is more or less the generic word for all the other titles. A King is a Hereditary Monarchy Governor, for example. And usually we use Governor as the title for a lot of translations rather than trying to figure out the right grand sounding title or the original word. The only titles that get used more or as frequently are King and Emperor, because King has come to mean "the guy in charge" and Emperor has come to mean "the guy in charge with lots and lots of land" rather than what they mean. The fact is that while we think of ruling/running/governing a city is no big deal, in the past the person we call a Mayor today would be an absolute Sovereign ruler. A King is just the title we came up with when one of those guys started beating the others into becoming loyal followers of theirs. And Emperor is just the title for when one of those guys did the same at the newer, higher level. So any title is fine as an "Absolute", but you gotta consider the reader too and how they're going to take and understand it. In which case Mayor or Governor may not be the right choice in terms of getting the desired picture across.
Part of the reason to use a known title is shorthand for telling the type of civilization this is and who's in charge. A Grand Duke sounds like he's in charge but just holding the seat for someone while an Archduke sounds like he lead a coup. And both of these, for some reason, sound more oppressive and evil than a King. It doesn't have to be that way, but it does "feel" that way so if you're going to use these terms I would suggest thinking on what they evoke in your mind about the civilization and how you might use that in your story. For example, the Archduke might give off the vibe he's a villain throughout the story and then he ends up being a hero, or he might act like a hero the entire story and then at the final moment reveal he's the villain. The first is a surprise to the reader while the latter makes the reader more suspicious and lets them feel vindicated when it turns out they're right.