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It's lesser-known, but the Battletech Advanced 3062 mod for Battletech from Harebrained Schemes is incredible. It adds so much it's almost an entirely new game (but still Battletech).

Carball for Unreal Tournament 2004.

They completely changed the game to be a proto rocket league game many years before rocket league was a thing.

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.

Fallout UK

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

I came across the Star Wars Helldivers mod. It’s basically Battlefront 3 but man it looks cool

I remember playing zombie master, a half life 2 mod on a lan party and being blown away by the mechanics. Also ,,the hidden,, was a new experience for me.

In zombiemaster one player plays against the rest and has a well... Real time strategy layout and spawns and controls different kind of zombies . The other players have to complete tasks and survive.

In ,,the hidden,, one player controls a almost invisible super strong rouge experimental super soldier against the special forces of iris .

Mechwarrior Living Legends is a mod for crisis that builds a compltely new game ontop of it and had a thriving multiplayer community for many years (not sure how activity is these days)

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.

On the Switch 1(or emulated), a Mario kart standalone mod call CTGP.

It's basically all the maps from all the MarioKarts and even a bunch of fan made maps on 1 game.

https://www.ctgpdx.com/

Fall from Heaven II. It's a fantasy themed overhaul mod for Civ IV. Has extraordinarily deep lore and unique game mechanics. I've never actually played the unmodded version of Civ IV. I only bought the game to play this mod.

If you haven't played Divinity Original Sin 2 with the Epic Encounters mod, you haven't experienced the game to its fullest potential. Give it a try if you already love DOS2 and would've liked to see more! 🙏

The Vanilla Expanded mod set for Rimworld

the multiplayer mod for BOTW

I've never played it myself, but Gary's Mod was all the rage back in the day.

Enderal

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.

I really liked the Custom Maps in Warcraft 3 online play. I used to play Hero Line Wars, Mauls and TDs, Dota, Legion TD among other amazing community made maps

Synergies for Torchlight II. It's like four extra games.

The "Vanilla: Expanded" series of mods for Rimworld adds an almost unbelievable number of new mechanics, almost all of which are intercompatible with most other popular mods. It's truly impressive how empty the default game feels after you play with mods for a while.

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

I was always astounded by hl2 mods.

There was one in particular that I really liked but it always crashed on me when I got even a little far in it. You spawn in a hotel and have to get away from the combine or something. I don't remember what it was called. But I really liked the idea of it.

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.

RogueTech for battle tech is amazing. Hundreds of hours into it.

Project Reality, a Battlefield 2 mod.

I think the Galactic Contention (Star Wars) mod for Squad looks very impressive, albeit I’ve never played it.

https://youtu.be/hl4m5HkRiUI