Gaming

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.

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.

Does Tabletop Simulator count? For board game people, mods are the whole point.

Arma 3. You could basically call the base game a platform for modding. DayZ as well as PUBG battlegrounds were originally mods for Arma.

not modding much, but what people do with skyrim is amazing

Skyrim

I don't know if improving is the right word, but the amount of transformative mods older games like Doom, Half-Life or Unreal Tournament (and not just shooters ofc) had, was wild.

Team Fortress started as a Quake mod, Counter-Strike as a HL mod, DotA as a Warcraft 3 mod.

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.

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

Cataclysm: DDA comes with a shitton of mods bundled and is superfun

Dragon Age Origins and Dragon Age 2, Minecraft, RimWorld, Stardew Valley, Raft, Civ 5.

Skyrim VR. The game without mods is probably one of the worst VR games ever, but with mods it becomes one of the most fun VR games.

There is no game getting worse with mods... Love my rimworld...

Sins of a Solar Empire: Rebellion has a lot of love and care put into mods. Being a sci-fi game, plenty of Star Trek / Star Wars / etc. conversions that do more than just reskin the models.

Warcraft 3 and the little thing called DotA.

Well, as you mentioned, Bethesda games in general. I always mod Elder Scrolls and Fallout games to a ridiculous degree. But some more:

Star Wars: Knights of the Old Republic has a great modding scene. I personally find "Start as a Jedi" essential for replays. And most of the mods (including that one) work on the excellent Android port.

Also, I love randomizers. I do a Symphony of the Night randomizer run at least 4 or 5 times a year, the Super Metroid x Link to the Past combined randomizer is amazing, and I'm finally going to dip my toe into Dark Souls randomizer soonish.

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

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.

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.

I love vanilla Terraria and Factorio but there are really fun mods out there that expand those games and don't let me play anything else!

Binding of Isaac is very improved by mods. Also, even if it's more plugins than mods, but every MMOs have a lot of plugins to custom the interface and make it more efficient for the type of gameplay you want

I already answered differently, but I want to put out that the STALKER fandom is held together by mods. There are everything from almost invisible bug mods to an entire standalone mod. STALKER is one of those franchises where modding just seems like such a natural fit to round out the world and it’s amazing how the vast majority of mods intend to support the tone of the game rather than just adding in the whims of the mod maker.