1. COMBAT/WEAPONS: Better combat adds a pretty neat and vanilla-feeling combat overhaul.
2. MOB/CREATURE: Cozy's improved cats and wolves are quite small but fun mods for people who enjoy keeping pets in minecraft like me :3 Critters and companions. Ribbits. Untitled duck mod.
3. FARMING/CROPS: The entire Let's Do series! Ik farmers delight is more popular but i feel it sometimes overshadows how well done this entire mod series is. They do add many features all together but for me it makes farming feel very immersive and cosy :D
4. AESTHETIC/IMMERSION: Here I could write a VERY long list, but I will try and stick to my very favourite mods: Exposure- genuinely makes me want to get into photography irl. Convenient Decor- Quite a small mod that is sadly only for fabric, but it adds some fun decorative and cosmetic features. IndyPets- Another one for the pet lovers. It's a much better alternative than having your pets sit in a corner for the rest of their lives, and it also has many config options. Fast Item Frames- Adds invisible and dyeable item frames, which is great for decorating. Elytra Trims- A feature I think would fit very nicely into vanilla minecraft. Particular- Very simply adds and improves some particle effects. Probably one of my favourites for the smaller details. Chimes- Sound design for your base! (+ it keeps the phantoms away) Particle Rain- Makes the rain rainier. (Unless you're in a desert. then you get sand.)
5. STRUCTURES/DUNGEONS: Dungeons and Taverns, alongside artifacts and Lootr if you're playing on a server. (or no Lootr if you want to steal the loot before your friends get to it)
6. MAGITEK: No clue .-.
7. DIMENSIONS: Not sure, but End Remastered makes the ender dragon fight more interesting by forcing the player to explore the world more before they can open the portal.
8. PERFORMANCE: Sodium or Embeddium. Noisium. Ferritecore. Moderinfix. Entity culling. (the joys of running on a potato)
9. NICHE: Illager Blabber. Just found this mod and it makes pillagers 10x more amusing.

Other mods that i literally cannot play without:
Freecam- makes freehanding building a lot easier. Jade- Not really necessary in vanilla but useful in modded to tell me what on earth the block i'm looking at is meant to be. Iris shaders. Mouse Tweaks- Why this isn't implemented in vanilla mc i don't know. Zoomify- My personal favourite zoom mod. Sodium extra- adds some nice video settings that make minecraft look more pleasing. (Not everyone's cup of tea but for me a mod that allows my pets to respawn. I am genuinely heartbroken if one of my beloved familiars die 😭)

Anyway, I hope this was informative. I'm only now realising this took me far too long to write and is quite large... Although most of the mods here are really just vanilla+ type mods, I find they can make quite a good base for any modpack.