Decline diplomacy: allows you to run through AIs turn without clicking no to hopeless offers. Especially important if you have a slower PC and want to do stuff while it runs through the end turn.
Cpecific's skill queue: lets you queue skill selection. Massive improvement of downtime of which there is too much. Only wish there was something similar for buildings.
AI general 3: Lets you have AI take over in battle. Nice if you want to spectate the great graphics also good to have AI do the mop up once you have secured victory. It has some flaws, is even worse than AIs AI somehow...but still great QOL
Better camera mod: lets you get really close to units to really see the deatils of the great models.
Land battles only (remove siege and Minor settlement): removes most walls, so instead of tedious sieges you can play open field battles. Also great for unit balance since siege favors ranged and really disadvantages cavalry.

These next few i also see as essential but they stray further from the QOL of the above and alter gameplay somewhat:
Varied AI Armies makes AI recruit faction specific sets of units rather than every faction bringing a bunch of infantry for your mages to murder. Creates more challenging and strategic gameplay.
Level 5 capital (or any level): every settlement has max tier main building; this speeds up the game significantly and lets you and AI field tier 5 units very early. Without it the first 50 turns is low and medium units and the game is basically over by the time you fight your first dragon.
Greater garrisons: adds units from many buildings to the settlement garrison has two significant effects; makes assaulting enemy settlements harder, often requiring two armies if enemy has an army there. This slow player down and prevents steamrolling the game. You still eventually become the greatest power if you dont play highest difficulty but it takes much longer to get there. The other effect is that you have a much easier time defending and dont lose settlements to AI swarming you with small armies.
Automatic immortality level 2 unlock: just a personal preference. This way losing a character isnt a great issue and you can enjoy the game more.

The game is playable without these but they fix most common gripes with gameplay and greatly improves the game imo

note; if you play a nonsettlement faction you may want to skip the level 5 capital and greater garrisons, as you dont benefit at all but will struggle against AIs benefit.