Grimacing with pain... I'm going to say that for the purposes of this question, I'm going to skip all the bugfix mods and the staples-of-everything mods like SKSE and SkyUI. I'm also going to skip the enabling mods like OAR and Base Object Swapper, because so much is built on top of them now that they're requirements.

When I mod Skyrim, once I have the base layer in place, what do I always include even though I don't need it?

True Directional Movement. This is so good that for me it's now a base mod. I love that third person mode both feels good and looks good.

High-Poly Head and NPC makeovers using it. Hard to un-see the clonk in the old heads once you see the nicer ones.

QuickLoot. Massive QOL improver.

Any mod that adds cloaks.

ENB Lights. They make the game so pretty.

Oh man, that’s hard to say

Wabbajack is pretty incredible, the ability to get a full working list with like zero effort is wild

As for mods themselves, I love Clockwork, the creepy emptiness of it is amazing

I have many more than these, but here are a few.

Sons of Skyrim - Probably the most impressive armor mod I've seen. While there are many high quality armor mods, so many feel more appropriate for a fashion show rather than Skyrim. There's nothing wrong with that, but it's nice to have such a quality mod that feels at home in the game. https://www.nexusmods.com/skyrimspecialedition/mods/68656

Skyrim Unbound Reborn - I have played the main quest once in my years playing Skyrim and haven't personally been interested in playing it again. (Save the world hero stories don't interest me too much) So it's nice to play as a non Dragonborn. https://www.nexusmods.com/skyrimspecialedition/mods/27962

Missives - Since I don't care much for the main quest mods like this give me more things to do in game. https://www.nexusmods.com/skyrimspecialedition/mods/17576

Jayserpa's Quest Expansions - Having more choices during quests is always great, which I always felt was lacking in vanilla. His other mods are great as well. https://www.nexusmods.com/skyrimspecialedition/users/5201727

Expressive Facegen Morphs - I always struggle creating a character I like and this provides more options for RaceMenu. Great mod. https://www.nexusmods.com/skyrimspecialedition/mods/35785

Ooh my first Lemmy post.

I'll mention my new fave mod that I can't imagine living without from here on:

Player Equipment Manager on Wheels.

This lets you quickly swap base outfits and add-ons to those outfits, complete with hotkeys and nifty menus. Everything about this mod just works, and it's pretty simple to get the hang of.

Ooooh, I have plenty! Years of modding have resulted in soooo many excellent choices, my personal favorites are: JK's Skyrim - https://www.nexusmods.com/skyrimspecialedition/mods/6289 Gives so much more life and detail to all the cities.

Northern Roads - https://www.nexusmods.com/skyrimspecialedition/mods/77530 A somewhat newer mod that really caught my eye. An overhaul of ALL of the main roads in Skyrim, changing the road material and adding props and details all around.

Sekiro Combat S - https://www.nexusmods.com/skyrimspecialedition/mods/79206 This mod adds a Sekiro-styled parry effect to melee blocking. A lightweight mod that can be added on it's own to add some flavor to Skyrim's somewhat bland melee combat.

Signature Equipment - https://www.nexusmods.com/skyrimspecialedition/mods/16190 Weapons and armor slowly get better the more they're used in combat. A simple one, but I end up getting attached to my gear, so something like this just gives more tales to tell!

High Level Enemies Redux - https://www.nexusmods.com/skyrimspecialedition/mods/32801 Adds tons of new enemy variants and scaling enemies. I love seeking out strong enemies and skyrim's difficulty kind of tanks after a certain point. This mod fixes that! Plus, this mod in tandem with the Sekiro Combat mod gives a real reason to learn to parry!

In terms of stuff that goes in the backend of the game, increasing the overall functionality I'd say SPID (https://www.nexusmods.com/skyrimspecialedition/mods/36869)

And also Base Object Swapper (https://www.nexusmods.com/skyrimspecialedition/mods/60805)

A lot of the mods that take advantage of these I find to be much better in terms of ease of use and compatibility for distributing new armors, abilities, weapons, etc. And a few older, script heavy mods like Immersive Armours and Wet and Cold have been given SPID versions if you're still interested in using them but wanted less script bloat to your save

Now that I've played with it, I also really like Seasons of Skyrim (https://www.nexusmods.com/skyrimspecialedition/mods/62861) with the Turn of the Seasons or Seasonal Landscape add-on. It's not too hard to install but for me it definitely helped me get better at modding, and in game in is really cool to see Skyrim in a totally new way. I would recommend you get some of those time passer mods like Time Flies (https://www.nexusmods.com/skyrimspecialedition/mods/39426) so that you can see the seasons a little more as this mod will allow time to pass while you do things slike read, smith, or sell stuff.