I'll make a list of some good ones to start with.

This let's you move thing around in the hand crafting que.

Hide the minimap if it's preventing entity data from being shown. Very useful.

These four just move various items into their own crafting category so they don't clutter up the the menu. This is very important if you have many mods that add items.

Makes it so tier 3 belts move at exactly twice the speed of tier 2 belts, and tier 1 at half the speed of tier 2. This let's you use splitters easier.

These three just make things look nicer.

Enhances the day/night cycle. Very customizable.

Auto repairs items that are in your inventory using repair packs.

Makes fuel inserters actually useful

Visual improvement. Makes the map easier to read.

These two make handling fluids much nicer and simpler.

Let's mods include ingame tutorials on how to use them. Sometimes, it's almost impossible to understand what's going on without it.

Makes it so you can navigate your base easier.

Adds important details to item descriptions that you would otherwise have to look up online to find.

Rampant

It doesn't really add much complexity, all it adds is suffering and pain (my last 6 games have lasted around 12 hours before my defences get completely overrun and i'm forced to start over)

But it all depends on what you want to do really, logistic train network LTN is a mod that lets you build a train network that works almost like logistic bots, that can be quite interesting to play with, and makes building megabases centered around trains much more manageable

Krastorio 2 is the recommended overhaul mod after a vanilla playthrough. It makes the game somewhat harder than vanilla, although I don't really know what it contains as I never played through it.

Industrial Revolution 3 is another good choice.

Or you can jump off the deep end like me and start a Angels+Bobs map, but this is quite difficult dealing with many byproducts and thousands of recipies. Also, you don't need many trains to beat it, I beat the modpack with 3 train lines, but if you want to scale up trains would be useful.