Mine would have to be:
SkyUI
Unofficial Patch
Simply Knock
SMIM
Blended Roads
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.
My only complaint about the SPID version of Wet & Cold is that the author also chose to rip out all the AI features, where NPCs come in out of the rain. I recently tried to recreate the full W&C experience with a collection of smaller mods, and I couldn’t find anything to recreate that behavior. It’s the kind of thing that will make me figure out how to write AI packages some day.
I think the popular AI overhauls might allow the NPC characters to do that, at least AI Overhaul (https://www.nexusmods.com/skyrimspecialedition/mods/21654) says that it gives AI weather behavior, so maybe that could help you!
