If you look at the settlement growth modifiers, there's a lot of things in your control - namely the buildings in the settlement and the tax rate. However there's one other modifier that is static throughout the whole game and no matter how hard you try, you aren't changing it without modding the game. That's the Base Farming Level.

Put Simply the Base Farming Level is how "good" the land the settlement is on.

Much of the content in this game is locked by the sizes of your city - you can only make the best units if your buildings are high tier, which you can only do if you have a lot of people, which you need growth to do that.

A farm in a settlement with a base farming level of .5% is going to be just as effective as a farm in a settlement with a base farming level of 7% - which means the best way to access the late game buildings and units earlier (without forced migration or cheating or any other exploits I'm not aware of) is to take provinces with high base farming level.

Here's a map I made after digging in the game files for the Base Farm Level of each Settlement in the game. This information can be found at: steamapps\common\Total War ROME REMASTERED\Contents\Resources\Data\data\world\maps\base\descr_regions.txt
In the file, the bottom number for each settlement is the base farming level. 1 =.5%, 2=1% and so on

Sorry the map is really badly edited, didn't feel like making it pretty - I just wanted a basic visual representation.

Gray = .5% - 1.5%
Dark Red = 2% - 2.5%
Red = 3% - 3.5%
Orange = 4% - 4.5%
Yellow = 5% - 5.5%
Green = 6% and up. these Provinces have their percentage added onto the map for quick reference. It can be a little small so click on it to make it bigger.

What I've learned from making this map is that - yes, Greece, Asia Minor, and the Nile Delta are some of the best provinces for growth in the game. Which I knew previously but the point of making this was to identify the exact provinces that had the best growth since I wanted to turn them into recruitment centers.

An interesting thing of note is that while the Eastern Mediterranean has some of the best base farming levels overall, The best provinces in terms of Base Farming level are only found outside of these high development areas.

The two provinces with 7% are: Patavium and Carthage
Corduba has 6.5%
and the rest of the green provinces - which are in the traditionally understood high growth areas - all have 6%

In case you're interested, there's only one province with .5% - and that's Nepte

Happy Conquering!