I'm still waiting on some of the total conversions to get more developed, so I tend to stick to QoL mods. Here are some of mine:
Community Flavor Pack
Extended Outliner
Clear Notifcations
In My Humble Opinion
More Interactive Vassals
Great Empire Wars
Prisoner's of War
In My Humble Opinion adds the ability to see what your current character links of a character you're looking at - just like in CK2 which is great for some quick RP
Prisoner's of War is a great small additional in which characters taken captive in battle don't just teleport to the war leaders dungeon and are instead held by the capturing army until the make it back to their borders.
What are your favorites???
I originally bought ck3 solely for elder kings 2. I prefer the original game over it now, but it'll always hold a special place to me for pulling me in, lol