HD Reworked Project - Reworks many of the lesser quality textures in the game.
Friendly HUD - Makes the HUD a lot more dynamic and potions/oils and so on can now be applied from the radial menu.
The Enhancement System - A lot more of the gear in the game can now be upgraded so that a sword or a piece of armor that you might like visually can now be used for longer
Volumetric Clouds over Skellige - Places clouds around the mountain peaks of Skellige, just like in Kaer Morhen and in the prelease footage.
Witcher 2 Gear - Bring over much of the armor pieces from the Witcher 2, replacing most of the common armors that didn't even look that good anyways.
All NPC Scabbards - Every NPC that carries a sword will now also carry a scabbard for it.
AutoLoot - Simplifies and speeds up the looting process
Ultra Grass 420 - Increases grass density and brings over the higher quality grass models from the Blood and Wine expansion to Velen and Novigrad
Weather Enhanced - Makes the weather a lot more varied
The Appearance Menu Mod - Lets you customize the look of Geralt and Roach to you hearts content
I also recommend checking out the work by the following mod authors, pretty much all of it is good stuff.
wghost81
Holgar96
KNGR
boostin4kix
skacikpl
erxv
DJ_Kovrik
rustine
CAPA14
Walteriusmaximus
Also make sure to check out
Asmodean778's list of personal mods
There are plenty other good mods too out there put listing all of them would take forever and reading such a long post would probably just get boring and I have to leave some things for you to discover for yourself

.
I am super grateful to everyone that has put in effort to make this already great game an even better one this past year, keep it up to all of you!