The only essential mod is the Network Addon Mod.

If you feel like searching for all the dependencies, I can also recommend the CAM mod, this mod adds a lot of different building assets and enables more Euro or Asian style cities (high density). Makes dense skyscraper areas more viable. Setting it up is a massive pain as you have to go search for additional asset packs, although I believe CAM should work without them (but then you don't get the asset variety).

I also like to include different sized service buildings (e.g. very large hospital) to support the higher populations with CAM.

Put in "hospital" (or "fire station") here:

https://community.simtropolis.com/files/

Pick one that looks good to you.

Honestly, you can just go with the Network Addon Mod, it is the only I would consider essential for everyone.