What is your favorite mod?

Terraria - Calamity.
Expands the game to 2.5x the content.
Absolutely amazing, would recommend buying Terraria just to play it.

I've never been one to mod games, I'm pretty vanilla when it comes to gaming. Play the base game and thats it. Rarely do i even try any dlc. With that said, I thoroughly enjoyed Frelancer discovery back in the day. It is a freelancer multilayer mod that brought a lot more content and enabled multi player once the official servers were gone.

Garry’s Mod
It’s hard to think of any one mod that got remotely close to changing a game the way GMod did for HL2/CS:S/source engine titles. I spent thousands of hours in GMod as a kid, it added infinite teplaya ility to the HL2 campaign, forums like Facepunch and PHWOnline became my second home. There was a ton of content to be loaded from there and FPSBanana, the thriving webcomic scene was truly special.

The mod that got me into modding still has a special place in my heart. It was the Shockwave mod for C&C Generals. Shockwave absolutely could have been vanilla if EA deigned to finish the Zero Hour expansion. It was just pure polished perfection for that game.

Gigastructural Engineering for Stellaris... the megastructures included in the game and DLC are great, but they lack a certain insanity. Plus the mod allows you to terraform any planet (even gas giants).

Better Than Wolves it's a minecraft overhaul that was started by the veteran game designer FlowerChild who was pissed at Mojang that they are adding useless features that don't improve gameplay loop (wolves being the breaking point). So he spent many years making Minecraft challenging survival and interesting tech game.
While I haven't beaten the dragon once in that game (not many people have) it still gave me hundreds of hours of entertainment.

Archolos, and it’s not even close. This is one of the best gaming experiences i’ve had in years
- Screw Gothic 3 and 4, this is the real sequel us Gothic fans have been waiting for! A genuine delight to play through, and the Polish voice acting was just as incredible as it was in Gothic 1 and 2, despite being a fan project! Waiting patiently for the Mod of the Decade edition to re-play it again and try out all the new things they add (very excited for the teased mage path!)

Enderal, a whole new game built ontop of Skyrim. Offers an entirely different setting, map, class system, enemies, spells, and has an actual story that you will not soon forget.

I'm going to have to go with either the Create mod or Apotheosis, both for Minecraft. Both of them feel like they could just be part of the vanilla game, and at this point it feels wrong and weird for me to play the game without them installed.

Sky UI. Not the same game without it.
- So mandatory that mentally I don't even consider it a mod.

There's a Sonic fan game I like to play called Sonic Robo Blast 2, built on an extremely heavily modified OG Doom engine with a pretty good modding community, and there's a level pack for it called Sol Sestancia that's just crazy fun to run through with the Neo Sonic character mod. Getting up to top speed to activate boost mode and trying your best not to slow down or stop so you don't lose it. So satisfying.

There are so many... I'll just go with a couple
Fallout 3 Wanderers Edition always comes to mind for making that game so much better to play and more stable than the base game. I wouldn't replay FO3 any other way
Elden Ring Seamless Co-op is what ER co-op should have been to begin with. God it's so good I put another couple hundred hours in to replay every inch of ER+DLC with a friend.

This was a tough one. But I'm probably going to have to say JSawyer Ultimate for Fallout: New Vegas. A continuation of the game director's own rebalance mod, it makes the game hard in a way that feels fresh and fair (unlike the baked-in Hardcore mode).
I like to pair it with my runner-up: JAM - Just Assorted Mods, for a more modern HUD and a sprint feature. Of course then you need the NPCs Sprint mod to rebalance combat and- you know what, all of the Viva New Vegas modlist while you're at it ;)

STALKER Anomaly with GAMMA on top of course.

I'd have to say thanks to SMAPI and Content Patcher for enabling so many Stardew Valley mods in the first place.
Also, Qwinn's Ultimate DAO Fixpack for allowing me to have a mostly bug-free experience playing Dragon Age: Origins. Mostly.

Most impressive:
Assetto Corsa Custom Shaders Patch / Content Manager
Tiniest mod which solves biggest frustration:
Heroes of Hammerwatch rejoin mod
- Yep. My answer is "CSP/Content Manager" then "every other Assetto Corsa mod." (Also worth noting Shutoko Revival Project.)
  
  Assert Corsa goes from an ok game without csp/content manager to an amazing game with them.

I want to highlight two mod makers, instead of single mods. lStewieAl and WallSoGB for their efforts unfucking New Vegas. Being forced to pick a single effort, the engine optimizations is probably the most impressive. Honorable mention to all the various script extenders that make so many fantastic mods possible in the first place.

Star Trek Armada 3 for Sins of a Solar Empire. Can't wait for STA4 for SoaSE2.

Sekiro - For the sake of Ashina

The Fallout London mod for Fallout 4 was incredible. It's like a an entire new game for free. They really captured the spirit of the Fallout universe. Better than anything Bethesda put out in the past couple years
- I tried so hard to get this one to work and it just... didn't. The intro played, vanilla FO4 started. I patched it again, vanilla intro started, crash to desktop. Even tried versions from Steam & GoG. Manually patched it, failed. Downloaded the patcher from GoG, failed.

Hoi4 player-led peace conferences. Because the game is literally unplayable without it
P.S: I know this isn't the type of mod you meant, but often the small fixes are the most important
- I wasn't excluding this type of mod when I asked my question. Small fixes definitely count!

!Wicked Whims!<
- Spoilers on Lemmy work like this:
  
  ::: spoiler Some words you want to put The spoiler text Blah blah more spoiler text :::

for Minecraft, the fabulously optimized modpack. gives me over 5x the fps, meaning I can make the game look fancier with higher settings

NEOTOKYO°
I haven't bothered with mods for years now but Neotokyo was by far my favourite. Apparently it's still played to this day by a group of fans but I haven't joined in. Also the soundtrack is amazing

I think it's really impressive what the community has done with Super Smash Bros. Brawl. Project+ is basically the current "flagship" modpack. It builds on Legacy TE (which is built off Project M), so there's loads of new content and more Melee-like gameplay.
Another Brawl mod I like is Brawl -. Its main thing is that it balances the game by making everyone's movesets overpowered.
Changing genre completely, I really like the Gay Awakening and Gay Fates mods for Fire Emblem: Awakening and Fire Emblem: Fates. The main thing the mods do is add new romantic and platonic supports to the game. The focus is, as the name implies, on adding gay and lesbian content, but it also adds straight content to a lesser degree (like for the characters that can only support with Robin or Corrin).
- Ooh I forgot about Project+! The only way to play Brawl. It turns it into possibly the best Smash game to date.

definitely dremor and close second is shitpostpolice. oh boy my lips are so tired.

Minecraft - mouse tweaks, Alex's Mobs, The Bumblezone
Morrowind - I forget what it's called, but the one that adds the mainland
Risk of Rain 2 - any character by Enforcergang. Especially classic sniper and rocket

You know, I actually think it's the miniblocks datapack for Minecraft. That's the only "mod" that is included in every single world I play in. Every other mod in every other game is kind of optional, but I'll never play Minecraft without miniblocks ever again. Not that I use them a ton, but they singlehandedly managed to actually get me exceited for the wandering trader again!

Antistazi for Arma 3

Any mod that allows me to play as Amy Rose in any Sonic game.

This is going to be a callback, but Ballistic Weapons for UT2004. It completely overhauled the gunplay in ways I’ve never seen replicated. I loved all the extra modes the weapons had and many took a while to learn how to use.
The devs went on to make Viscera Cleanup Detail, so good for them, but I did always wish they made a FPS.