The mod for Skyrim that turns all the dragons into Macho Man Randy Savage

Project M and P+ for Brawl

Okay so this requires a little background.

In warhammer total war they have been hinting at the release of Dogs of War (Under the umbrella of "Southern Realms") since release of game 1. It's been over a decade with nary a peep. In fact they recently started patching hints about Dogs of War out of the game.
Anyway people got tired or waiting so they made a functional faction mod for them with numerous special characters, nore units and unit variety than most official factions, and its compatible with most of the other big mods. Mods that add extra special locations, characters and faction mechanics will usually also just add stuff to the "Southern Realms' because why not.
Because another mod also added the same character as this mod if you have both you get a special quest to kill the guy "pretending to be you" with associated lore pop ups.

Brutal Doom with the Hell on Earth/Extermination Day mappack, or Doom 2 Reloaded/Hellcore 2.0

Ashes 2063, which is now a standalone mod for Doom 2

Duke Nukem Forever 2013 mod with the DLC

Jagged Alliance 1.13 with Arulco Revisited or the 1.13 Urban Chaos mod (this one is actually hell to get working but so worth it)

Half-Life 2 MMoD and the FakeFactory 2013 Cinematic Mod, with the NPC models and graffiti removed

Brita's Armour and Weapons pack for Project Zomboid (version 41.78), though it may be retired now.

ArcCW weapons pack for Garry's Mod, mainly Urban Chaos/Decay/Renewal and Gunsmith Offensive, just to have loads of guns to use in sandbox roleplay or playing through the Half-Life 2 episodic or other custom campaigns.

FTL Multiverse or Captain's Edition

Fallout 4 Modern Firearms mod (though the people making it are turbochuds)

RuneLite

Unreal Tournament 2004 Ballistic Weapons pack

OpenTTD with RIMS/FRIMS Maglev train set and the Chinese trains, complete with the CR380 and CR400

I literally only own medieval 2 total war because of the mods. I’ve played through the vanilla game I think exactly once

Civ 4 BTS has plenty of fantastic mod packs that can transform it into several different games in one. Dune Wars: Revival, Fall from Heaven 2, Caveman2Cosmos, but if I had to pick one? Even if I hadn't played it in a while? History Rewritten. Keeps the feel of the base game while heavily expanding on it and really polishing it out, including dozens of new civs, all with multiple leaders, and each civ gets their own unique unit art, city assets, and leader themes. Peak flavor

Long War of the Chosen for XCOM2

There's several big factorio mods I've enjoyed, Bob's/Angels, Pyanodon's, off the top of my head

Project M

Oscuro's Oblivion Overhaul was the first really big mod I installed way back when and it's what got me into fucking around with mods in the first place. It will always hold a special place in my heart.

original tekkit and aether mod. The real og’s of minecraft modding, they hit a high so good Ive only ever been let down ever since

was in the same boat and i got into gregtech new horizons lately and it hits the high. probs most fun i've ever had in minecraft

I forgot about Project M, but purpleworm's got that covered.

Another one is KSP: RO/RP-1. RP-1 is a complete overhaul of the game combining a decade of nearly all of the greatest individual mods into a nearly cohesive whole. Replaces the game's toy solar system with a full scale one. It changes the campaign to re-create the space race, with the tech tree filled with real engines and technologies from the US and Soviet space programs. Introduces real fuels mixtures with realistic properties which must be used with the correct engines and appropriate storage. Introduces line of sight and range limitations to radio communication. Substantially improved aerodynamics simulation. Substantially improved physics (necessitating spin stabilization and making spin a persistent property of airborn craft). Nerfs the original game's over-powered gyros and RCS. Simulates fuel ullage. Research takes time. Construction takes time. Astronaut mission training takes time. Astronauts retire. Procedural tanks, wings, fairings. Custom decals. Engine failures as a gameplay mechanic (forcing you to design something that is practical to build several of, if necessary). Life support systems for crewed missions. Pilot vulnerability to G-forces and loss of consciousness (and control). I'm sure I'm missing some things too.

While they are not included, it is compatible with Principia, an n-body gravity simulation enabling lagrange orbits, as well as kOS, the "Kerbal Operating System" which allows you to write control programs for your crafts. These could be as simple as launch sequencing, to sophisticated PID controllers, custom autopilots, and automated station-keeping procedures for satellites.

Also the OG Celeste Strawberry Jam Collab. An enourmous community project featuring the work of over 350 contributors. It contains 116 levels spanning over 1500 individual rooms, as well as NINE HOURS of original music. Absolutely incredible.

Requiem overhaul for Skyrim was my jam, makes it more immersive and touches just about everything in the game, from animals only dropping pelts / claws etc to trolls having hyper regeneration unless you use some kind of fire to negate it. Draugr are virtually immune to anything but silver weapons and fire. The damage is all turned up by magnitudes for both the player and npcs, so getting hit by a couple arrows can down you, a two handed axe will potentially one shot you in light armor. Would usually have to run around killing bandits for a while just to survive the starter dungeon.

Long war of the chosen for xcom, God I sank an embarrassing number of hours into the game just because of that mod.

I've been enjoying most Vintage Story mods, especially Expanded Foods/Cullinary Artillery. Though the amount of AI and vibe coding tolerated amongst VS mods is annoying with no required disclosures, so the game crashes semi often with all the mods I use now. There's also no complete mod overhaul of the game because it's sorta young and I'm pretty sure it originally started as a mod for minecraft in the first place (never played modded MC so idk).

Also been wanting to try Turyle WoW for a while, but haven't started yet

Edit: forgot to say I used to play a ton of Battlefield 2 as well and I just checked in on project reality and they are STILL RELEASING UPDATES??? It's been like 20 years holy hell

Anything that Kaze Emanuar does is the best mod to ever exist. He made Super Mario: Ocarina of Time and is making/made his own whole refactored (like remastered the game engine) mod of SM64.

But then there's a simple one that remove Gleba from Factorio that made my friend laugh so I like it.

Then Minecraft, Terraria, and Slay the Spire have absolute cinema for mods but I haven't explored

The Last Days of the Third Age mod for Mount and Blade Warband is the best Lord of the Rings game I have ever had the joy of playing.

The Sith Lords Restored Content Mod for KOTOR2

You can actually go the HK 47 factory which there's loads of hints about in the base game which I spend hours trying to find playing it on console as a kid, and has a bunch of other cut stuff like way more weapon/armour/lightsabre mods

• Dark Future for Cyberpunk 2077

• Basically every mod made for Kerbal Space Program 1 (i loved that c. 2016 modding community so much i was like the resident cheerleader/beta tester/booster for the modders everyone was so sweet and nerdy)

• Kaiserreich for HOI4 even though i've evolved beyond it actually helped me conceptualize a leftist revolution, finally see leftists as good guys, and forced me to deep dive into so much 20th history of the left

• shoutout to the Star Trek Stellaris mod even if it ran like shit

• some fallout 4 mod that let me aesthetically build the Minutemen into an primitive anarcho-communist faction. i almost built a mod for that game to give it an actual story

• so many others: mods to me are like fanfic is to my partner it's my special interest but i am too sleepy/high to remember any others because if I did there would be like 100 entries.

JK+ for Jedi Academy! I spent probably more time playing that mod on multiplayer servers than any other video game when I was a teenager. Jedi Academy had a ton of great mods too, great single player campaigns, great multiplayer maps and skins, just an absolutely perfect modding scene.

Now while that's my most played mod, I think my favorite mod is Movie Battles 2. MB2 is basically a hero shooter but without predatory monetization - you pick your classes each class has different skills so you can pick a different load out there's some modes that recreate scenes from the movies some modes that are like free-flowing where you get a single life, and it has by far the best depiction of lightsaber combat in any Star Wars game, and it manages to do that without making the Jedi super OP. 10/10 game, the servers are still up, although every time I check in out of nostalgia there's people using slurs in the chat, so...

Anyone remember The Specialists mod for old school Counterstrike? Was really fun being 12 and doing matrix shit. Flipping off walls and slow mo dives and stuff. That's the first thing that comes to mind for me, although I'm sure I messed with some other stuff over the years.

Do rom hacks count as mods? If yes, then I'm currently having a TON of fun with Donkey Kong Country 2: The Lost Levels, and am eagerly awaiting the 1.0 release of DKC4.

original tekkit and aether mod. The real og’s of minecraft modding, they hit a high so good Ive only ever been let down ever since. Dont forget to delete you META-INF folder!

If we are going for packs, then the original Feed the Beast map + modpack where you need to advance through the mods on a floating island and feed items to the beast contraption for rewards.

Individual mods red power 2. So much of that mod actually ended up in vanilla in some form. But it's still not close.

Do hacks count? If so mother 3 English translation

Sonic Robo Blast 2: Persona

It's a Persona mod for a Sonic mod for Doom that converts the game into a fully functional online multiplayer recreation of Tartarus from Persona 3 but with sonic characters. It is genuinely awe-inspiring that it exists

The Open Cities mod for Skyrim made the open world and cities one cohesive area without loading screens. Such an improvement to the feel of the game.

The Kung Foo mods for Max Payne were awesome, especially at a time in my life where I was first experiencing Hong Kong cinema.

Total Annihilation had some insane mods that turned the game into an apocalyptic playground.

Anbennar is an amazing mod for Europa Universalis, the world is so much more interesting without Europe

all the UI mods that make strategy games playable

https://www.nexusmods.com/skyrimspecialedition/mods/11802

Basically, you get a museum to put all your shit in. It rapidly becomes the focus of every playthrough I do.

Desert Combat for Battlefield 1942

Tamriel Rebuilt, Project Cyrodiil, Skyrim Home of the Nords and the other province mods for Morrowind. Better quests, items, landscapes and dungeons than vanilla bethesda stuff and it's all free.

Fx mod for homeworld 2 and remastered. Complex mod for home world remastered. Empire of fairies mod for homeworld 2 Path to victory mod for homeworld 2.

It's a shame the company didn't include engine optimizations in the remastered, or backwards compatibility with mods, despite using the same engine. Not even the unremastered re release of homeworld 2 is compatible with most mods. essentially killing the mod scene.

I always liked the Mount & Blade total conversions. The base game was already a jank mess so it didn't really matter much that the mods were fucked too (see: fully-automatic elf archers in PoP). Still waiting on similar stuff for Bannerlord, sadly.

Tabletop Sim has so many mods that let you play just about any boardgame. Shout out to Spirit Island and Heat.

Total Warhammer 3 has a good number of mods, and I basically never run it vanilla anymore. Mostly use stuff like Victory Conditions Overhaul to get more varied (and actually achievable) Immoral Empires victory conditions, or Warband Upgrade Ultimate to be able to promote units to other ones at higher tiers.

Content Manager for Assetto Corsa

Matrix Lobby for Max Payne

Normally I don't ever really fuck with mods outside of unofficial user patches, gameplay fixes or launchers, that said though I did download sonic r on the pc and paired it with a total gameplay overhaul mod which worked smashingly. Oh and Diablo 2 LOD, pluggy stash mod is a must have.