Mine would have to be:
SkyUI
Unofficial Patch
Simply Knock
SMIM
Blended Roads
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