Disclaimer

Cue in the usual disclaimer! These are the modders that took center stage in 2019. Quite a few did not make the list because the list has to stop at some point, but there are many more modders whose mod(s) I download immediately and forever use.
Modders are in alphabetical order because there is simply no way I could ever rank them.

AxeMagister/Archimag

The 4NM_ series are not for everyone but between 4NM_Training, 4NM_Combat, 4NM_Magic, 4NM_GMST, 4NM_Armorer, 4NN_Randomizer, you get yourself a whole new gameplay experience. The 4NM mods are maybe less polished than others and could use more customisation (as is often the case with large overhauls) but if you want a challenge and fun new mechanics, this is it. And that's only about half of the mods from AxeMagister this year. I use HardTrade, Beauty Scrolls, DragonDoor, and Antiwitcher in addition to NM4_Random.

DarkElfGuy

Ok, all right, DEG has not published any mod in 2019 BUT! He is a modder and with 60+ videos showcasing mods on YouTube in 2019 and yearly modding events such as the Morrowind Modding Madness he is definitely one of the most influential members of the Morrowind Community. Some would say he is one of the driving forces in the Morrowind revival we've witnessed these past few years.

Gavrilo

If you have endorsed a mod dealing with dialogues in 2019, chances are it was one of Gavrilo's. Between brand new factions (Census and excise Office), new quests (Bounty Hunter Bundle), much needed RP options for the main quest (Main Quest Overhaul), and countless other mods, you get quite a few hours of new Morrowind content. You know me, I love me some lua goodness, but I am also thankful for modders like Gavrilo that write quality quests for us to enjoy.

Greatness7

While Greatness7 has not published many mods in 2019, he pretty much had his oar in every great mod this year, either in a collab (MWSE Graphic Herbalism with Stuporstar, Tales of Ald Velothi for MMM) or quite simply by helping modders in the Morrowind Modding Discord with scripting and even developing modding tools.

Hrnchamd

Again, this is a modder that makes our modern Morrowind experience a reality with his work on MGE XE and MCP but also mods that we never thought possible such as Perfect Placement (bye bye towers of Pisa made of junk in our house), Pincushion (hello arrows in the knees) and Weather Adjuster (can Morrowind get any prettier?)

Merlord

It is no secret that I am a great fan of Merlord's mods, and last year, he certainly delivered: 15 mods to make Morrowind more challenging, more realistic, more balanced and more fun. My top 3 this year: Kill Command to make companions and summons attack a target of your choice, Realistic Repairs, because no adventurer would strip in the middle of a Sixth House dungeon and bang loudly on their armour, Character Backgrounds to make sure each new character is played differently than the last. But let's face it, I pretty much run every mod Merlord ever released.

NullCascade

Mods by NullCascade in 2019 include Last Witness and Give a Gift, two mods I keep in every MW install. Beyond that, we forever owe NullCascade for his work on the Morrowind Script Extender. I know, I know, MWSE nightly build dates back to 2018 but Null has updated MWSE with new functions as fast as modders came up with impossible ideas, pretty much ushering Morrowind into a new era of Modding.

OperatorJack

Magicka Expanded is possibly the most exciting mod of 2019 in terms of what it does and what it will do (before 2090): new spell effects (summons and teleportation, read minds and control the weather, whirl spells by swinging your wand, control your fireballs, turn items to gold... need I go on?), new vfx and a framework that other modders can use. I am also a big fan of Security Enhanced a highly customisable mod that forever replaced Fligg's good old Handy Equipping.

Remiros

Naturally, Groundcover comes to mind but also a good number of improved or new models (Nordic Mesh Improvements, Hunter's Mark - A Marksman Mod), as well as many aesthetically pleasing locations to discover (Foyada Mamaea Overhaul and Mawia Overhaul).

Honourable mentions

Abot

Abot has given us smart mods in 2019: Smart Maps, Smart Helmet, and Smart Ammo. Then of course, there is his continued care for all-time favourite mods such as Water Life and Silt Striders to only name a couple. I cannot recommend his website enough: in-depth articles about modding, patches for mods, less known mods, and the very useful search engine for mods.

Half11

Sure, Patch for Purists was first released in 2017, but this is a must-have mod that receives regular updates. In 2019, Half11 released Yet Another Guard Diversity, my favourite guard mod so far.

Mort

Because I will never play Morrowind without Quickloot or Mort's Ultimate Leveling Experience. The former lets you see what an unlocked container... huh contains and the other brings the comfort of mods like GCD, Madd and MCC Leveler but sexier and more stable. Also of note, Mort's update of Illuminated Order, one of the best quest mods out there.

Rytelier

Aesthesia Groundcover is one of the most endorsed mods in 2019. Muse (released in 2018) lets us mod our Morrowind Soundscape like never before, especially with the 2019 update. Modular cave set is used by other modders to great effect, Tamriel Rebuilt - Original Soundtrack is a must-have for anyone enjoying TR and Cinematic Camera - free smooth camera mode is pretty darn cool (that's what lets DarkElfGuy make those beautiful, smooth flyby videos).

TelShadow

Mostly known as the modding AI and for good reasons: no one can build faster than TelShadow. 2019 has been a very prolific year with a great many dungeons, brand new or improved vanilla locations. I'll link you to TelShadow's page because linking each mod individually would make my word count explode.