Gaming

Definitely RimWorld. There's so many mods that improve the base game. From QoL mods that make you wonder why that isn't default in the vanilla game, to mods that complety overhaul the actual win condition. Just overall a really fun, replayability, frustrating game.

Use mods though. It'll make it better. Check out p-music mod while you're at it.

FTL has complete overhaul mods and expansions that can make it seem new even now.

Definitely Minecraft, you can turn it into a completely different game

Stardew Valley is so much better with mods. Less grinding, more fun. I also like the mods for Skyrim and Minetest a FOSS Minecraft-like game.

X Com2 has amazing mods. Most of the content available for Battletech is from mods by like 2 orders of magnitude.

I won't even play beat saber without the mods. Half Life Alex has full games released in the engine as mods too!

Minecraft has a pretty solid vanilla experience but the depth of things you can do with mods is pretty insane. I’m playing a pack right now that basically turns it into a rogue like dungeon crawler.

ARK: Survival Evolved. There are no corner pieces for sloped roofs without mods... I like a good sloped roof, and a building that's not just a square/a-frame.

There's a ton!

Project Zomboid mods are fantastic. There are insane amounts of them, and they add so much to the game.

Games like Skyrim and Fallout 4 become essentially infinitely replayable with mods. There's a tool called Wabbajack that essentially makes "modpacks" a thing for these games (and several others), and it's amazing. You can install and configure hundreds of mods in like 30 mins.

One of my personal favorite mods was "Hyrule Conquest" for 0 A.D., which is a complete overhaul of the game that turns it into a deep, lore-friendly Zelda RTS. It's still being worked on and it's still super high quality. The dev made some major changes to the play style that a lot of people love, but I personally strongly preferred the old style. It was more like Age of Empires before, but now it's more like Battle for Middle Earth and I don't care for it. If that's a selling point for you, it's very much still a great mod with a lot of love and passion put into it.

Terraria, is still an excellent game but the Mod loader makes it tons better with plenty of feature mods and quality of life plus it is available from steam so easy set up.

Farming Simulator 22 and Snow Runner are both just absolutely different games when you add mods. The community is pretty active and the mod browser is built right into the menu, even on consoles. You can literally make your own game using mods. Without them, it could grow tiring after a while.

I really needed mods for The Witcher 3: Wild Hunt or I would have probably never finished the main story.

I had to get the mods that would autoloot and give unlimited inventory space, otherwise I would have minmaxed the game to tediousness by spamming pick-up and sorting through loot to keep the weight limit.

After getting the mods I could just focus on the story and gameplay without worrying if I'd maximised all my looting.

Assetto Corsa. So much new content, of amazing quality.

Valheim! A million and one mods to revitalize the game experience, and more than anything else the Valheim VR mod makes it an entirely new game! I’ve got 360 hours sunk into it with my 2 friends.

Civ 4 had a great modded scene. The Colonization remake/spinoff in particular has a must-have mod in the way of The Authentic Colonization. The main game, though, had loads and loads of incredible mods. My personal favorite was the Ryse series of mods, which tried to more accurately model the rise and fall of civilizations via various mechanics. I have a lot of hours in the random map variant of it, Ryse Rand.

7 days to die. The base game is in an...interesting state.

The Sims 4 is basically almost unplayable to me without custom content and mods and community made bugfixes.

Tabletop Simulator and Garry's mod is all modding, they would be very boring without it. (Tabletop might have the DLC addon content worth playing).

Cities Skylines fixes many of the broken aspects of vanilla and adds things to the point that Paradox added some of them into an enhanced console edition.

Slay the Spire, it's already an amazing game to begin with, but mods allow it to be absolutely insane with customization and cards. Even multiplayer is supported (Together in Spire) and works quite well!

American/Euro Truck Sim has a multitude of mods that add great stuff and even more realism to the game.

Call of Duty Black Ops 3 is more or less a bog-standard shooting game without mods, but the number of crazy, funny and innovative maps for zombies gives it an insane amount of replayability.

@SevenSwell EU4's modding scene vastly improves the game, with a variety of total overhauls available. Same goes for most Paradox games actually - the modding scene is usually well taken care of.

But I'd be lying to myself if I said anything other then Skyrim AE.

Skyrim! The modding community literally saved this game.

Skyrim. Still going strong after a decade

The original Doom for sure. I don't know if they count as mods since they're technically running on a modified version of the engine, but there is still a massive community making maps, mods, and even new game modes for it. Most recently the "MyHouse.wad" map for it has exploded in popularity. I've had so much fun over the years with doom mods, it's a treasure trove that most people have no idea exists.

There's even entire new games built on Doom. Sonic Robo Blast 2 Kart (stupid name, I know) is a fantastic free open source karting game with a decent community, and it's technically just a doom mod.

If anyone here is going to play a single doom mod to see what's up, I recommend Doom: The Golden Souls Remastered. It's good fun.

Rimworld. The Vanilla Expanded mods alone have more content than the base game + all the DLCs

The Steam version of Sonic Adventure is a bad port of a bad port, but just downloading the mod manager itself undoes one of those bads and the Dreamcast Conversion mod undoes the other, and that's not even getting into the crazy stuff people have done like hack Klonoa and their mechanics into the game

Factorio all the way. Get bored of finishing the game, or crafting the perfect megabases? Prepare for mods that can take 1000s of hours to finish. Perfection.

As always, the factory must grow.

Project Zomboid. Lots of mods that add new stuff (cars, weapons) and quality of life improvements like "Has Been Read".