Gaming

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

minecraft for sure

Another vote for RimWorld, though I want to point out that the game is perfectly playable without mods, too. It is not uncommon to see someone with hundreds of mods, but the vanilla game is just as fun. People have logged thousands of hours without mods.

Late to this post but to me it's Minecraft. It has such an insane amount of replayability and can be turned into a totally different game depending on the mods and whatnot.

Aside from the obvious minecraft... Arguably I'd say Factorio. They have a robust, feature-rich modding API built into the game that allows for relatively easy, wide ranging game play mods to be made very stable, and the number of mods has exploded as a result. The base game is amazing, but mods exist that quite literally triple the amount of game play and in some cases completely overhaul it into a totally new game. The support is amazing, and I wish more game companies could operate as efficiently as Wube does.

RimWorld. It's amazing what people can do from QOL to whole new factions or weapons. Amazing and the dev is very helpful during updates to try and not break mod support. Just blown away.

CounterStrike started as an Half-Life mod

Skyrim

I know it's not as rich or developed as the Fallout or Skyrim franchise, but my fun little time waste Stardew Valley is greatly enhanced by mods.

The base game is incredibly fun and simple, but even the most basic mods enhance the "quality of life", making some of the more surface time eating elements easier. But then you have entire new world maps, NPC's, quests and full DLC-esque mods such as STV Expanded almost a must-have for long term players.

As an FYI, I'd highly recommend the game to even the most hardcore gaming aficionados. It's refreshingly amusing and low-key, so when Elden Ring has you ready to throw your controller at the TV, it's a nice mental break.

Plus the guy that created it learned how from scratch and coded the whole thing solo- the source, the sprites, even the music. And every upgrade and addition he's made over the years, easily 2-3 DLC's themselves, he's given away for free. Support indie devs!

I played vanilla Stardew Valley once, and now I'm on my second go around, and I've installed a bunch of qol mods so that I don't have to keep checking the wiki and my collections to see if I should keep or sell this item I got because I don't know if I need it for an achievement, and now I have a notification that it's an NPC's birthday and I can just check to see if I own or am carrying an item they have. It just makes the game less stressful for a completionist like me.

Some I've modded besides what you said (Bethesda RPGs would be like, 8 games or something for me) would be Cities Skylines, Deus Ex, Factorio, Stardew Valley, Mount & Blade, Terraria, S.T.A.L.K.E.R.: Shadow of Chernobyl, Stellaris, Oxygen Not Included, Project Zomboid, Darkest Dungeon, Kenshi, Battletech.

KSP (Kerbal Space Program) with mods changes sooo much

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.

Just from a percentage standpoint, the Frackin Universe modpack adds so much to the Starbound experience which is otherwise relatively simple and something you only really play through once. There's like two or three extra Starbounds' worth of content, although that can make it overwhelming when starting out

Minecraft is an obvious answer of course, but there are some other really good games out there that get made much better with mods. Some of my favourite examples of this would be Star Wars Battlefront II (2017) with the cool hero mods and bot mods, and then Ghost Recon Wildlands with the amazing First-Person mod.

Doom 1 and 2 would be a lot less interesting without mods, some of which change the game rather dramatically.

• Project Brutality turns it into a modern-ish shooter.
• Guncaster replaces the usual protagonist with a spell-casting, oversized-pistol-slinging dragon.
• DemonSteele replaces the protagonist with an anime character.
• My House and City of the Damned: Apocalypse turn it into a horror game.

Without mods, these games would have just been historical footnotes, not something a significant number of people still play.

While not "saved" by modding, Kerbal space program is certainly not as interesting.

Not seeing anyone mention Crusader Kings yet (unless I'm blind)

Gameplay isn't for everyone but full conversion mods let you experience basically any world you want

Fallout New Vegas for sure, although many of the issues were fixed by the devs over the lifespan of the game. Mods are pretty much vital to run that game properly, however. Many issues with the engine and many core bugs have been fixed by the community, and it's honestly something where once you play with them fixed, it's hard to go back

Assetto Corsa. Outlived it's shelf life by many years, and holds up to racing sims with much more modern engines, all thanks to the incredibly dedicated modding community.

Space Engineers the base game is great, but it comes to life with mods.

From simple things like a more immersive way to paint blocks or customised UI to to hole NPC Factions, new Weapons, dekorative Stuff, all the way to hole new physics systems like Water with flotation, Aerodynamics or re-entry heat.

I rarely play my games with mods, this game is one of the few exceptions. I even went so far as to create some very minor mods my self.

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.

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

Half-Life 1

I have spent many happy hours playing GW2 and ESO. Mods have improved the GW2 experience significantly, but the game is still playable without them, if somewhat annoying. ESO is completely unplayable without lots and lots of mods, at least for me. I'm so directionally challenged that I gotta have my minimap.

I cannot speak for the entire S.T.A.L.K.E.R. series, but at least the first entry (Shadow of Chernobyl) is, while still very much playable, just so much better with a couple mods. And once you're familiar with the core gameplay, adding huge mods like Anomaly or G.A.M.M.A. (which itself is a modpack of Anomaly) adds many weeks worth of replayability to the game. Not even mentioning how good the game can look with these mods that touch it straight up into 2023. Of course, here and there you'll still notice that it's based on an older game, but it can easily compete with AAA releases of the past couple years in terms of atmosphere and immersion (especially the sound design).