The ACE mods have single-handedly kept Arma 3 alive for over a decade. They add so much complexity to the game with so many options for customization by both mission makers and players. It’s amazing to me how much difference the ACE modset makes to the game.

There are a TON of legit and amazing S.T.A.L.K.E.R mods like Misery, but my favorite is Anomaly. The mod team took assets across the three original Stalker releases, took alpha/beta builds, and combined all of these with a ton of other mods to make an ultimate OG stalker trilogy playground that is open and full of immersive experiences. Almost to the point where you can randomize starting characters and basically never play the same game again with how the NPCs have full independent routines.

Of course this means that a random quest giver can totally just get zeroed out and die before you ever even meet them, but that’s part of the experience lol.

Cursed Halo.

I believe it was a passion project by one guy that took 5 or 10 years but holy shit it’s a masterpiece.

Gekokujo mount and blade.

They practically rewrote the game from scratch. There is a story-mode.

There was also a minecraft mod that functional programmimg based magic system but it was interesting more than impressive.

Vampire: The Masquerade - Bloodlines Unofficial Patch by Wesp5.

That game was already broken at launch, and has become more broken as operating systems advance. Furthermore, Activision shuttered Troika almost immediately after VTMB was released, so there has really never been any official support. But, Wesp5 has been working on the Unofficial Patch since shortly after the game came out in 2004.

It fixes the game to a completely playable state, which is a miracle for VTMB. He basically just finished the game for them.

Furthermore, there was some controversy over some of the changes that the patch started implementing, so he split the patch into two versions. You can download the True Patch, which fixes the game to a playable version of its release state, but Wesp5 also offers the Plus Patch, which restores all content cut from the game before release. He reimplemented character backgrounds, real-world weapon names, multiplayer, quests, locations, music, and the list goes on.

It’s arguable if the Plus Patch actually represents the original vision of the creators, since we don’t know why all of the content was cut. A good bit was cut for time, but there were some things that were dropped earlier in development. Either way, the Plus Patch went feature-complete last year, so what we have now is a fully working version of the game containing absolutely everything that was ever planned during its development.

I don’t know if anything like that has happened before, and the only other thing like this that I can think of is the Saints Row 2 patch that IdolNinja was making before he died.

Mods that add stuff are great, but it’s hard to beat a mod that lets you play one of the greats.

Star Trek New Horizons for Stellaris. Best Star Trek game I’ve played.

Excessive Plus, a Quake 3 Arena mod

It’s a mod where the weapons are extra powerful. Making for some interesting physics like using the weapons for jumping and one shot kills. Even with all the madness of super weapons, it had great gameplay and competiveness.

The community lost the steam and now there aren’t many players last time I checked which is too bad because it was a great mod.

A movie with the mod: https://youtu.be/-qIvgdDznb8

Strawberry jam for celeste. It’s a huge collaborative project that introduces 100+ levels (base game has 25) with a really wide range of difficulties, graphical styles, new mechanics, original music and ideas on par with the quality from the base game. Definitely a must play if you like 2d platformers

DSfix

Single-handedly made an awful PC port playable.

For me personally it has to be certain rom hacks from before the era of decomps and creation kit style tools.

I was fairly active in the gen 3 Pokemon hacking scene on the pokecommunity forums back in the day. And hex editing and trial and error was one thing but real wizards were the ASM hackers that could completely out of the box stuff like adding in new move effects or character customisation functionality or day and night systems, to games that had none of these.

Those were actually the trailblazers that led to all the decomps we have today that have made it so easy.

Since someone already mentioned Fallout London, then I’ll recommend Shutoko Revival Project for Asseto Corsa, a full blown open map in the speedways of japan with online support and even functioning traffic so the illegal racing isn’t a breeze.

Can’t recall the name but there was a Halo PC mod which was an entire fan made campaign.

Black Mesa

People who improve Bethesda’s in game menus are awesome.

Half-Life: Firearms was sick back in the day.

Removed by mod

Witchery for Minecraft. Start out, building your altar room, find as many unique living things as possible and put them within 100 blocks of your altar. Mid game, make sleeping tonics to visit the spirit realm and try not to die from nightmares. Create voodoo dolls, and other nice high level gear, craft some powerful pvp oriented spells. Late game create a ritual room to summon demons to trade with. Don’t forget your protection circles because they move really fast and kill you almost instantly. Late game, become a vampire or werewolf or hybrid. Become nearly invincible outside of fire and sunlight.

Mekanism, an extensive tech mod for Minecraft that could quite happily be its own game.