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.

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

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.

Warcraft 3 and the little thing called DotA.

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.

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 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.

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.

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

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!

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

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.

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

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.

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!