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.

OG Diablo II is still thriving and kept alive thanks to some amazing mods, such as Project Diablo 2. Improved graphics, wide-screen support, and tons of QoL changes and additional content every few months keeps the game fresh and arguably, better than all other official iterations of Diablo out there.

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).

I don't know if it counts as mods but I love playing Wind Waker randomizer :3

Mount and Blade Warband! Base game is just ok, but mods like Diplomacy make it way better. Then you look at Prophecy of Pendor or Perisno and you've got basically an entirely new game with the same engine and it's awesome. I have hundreds of hours of Mount and Blade, and probably only like 25 of them are vanilla.

TES3MP was an amazing experience for Morrowind.

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.

All of the older Grand Theft Auto titles were saved by modding originally (ignoring the remade definitive edition trilogy of course) GTA III, GTA Vice City, GTA San Andreas, and even GTA IV were all released for older hardware and much different technology at the time they were released, so there's some weirdness getting each of the games to run without issues or well on modern hardware and modern Windows. GTA IV specifically has a notoriously bad PC port that is at times hard to play without any mods or community made utilities.

All four games are substantially better with mods, from small things like restoring the original soundtracks for each of the games that have since been patched out due to licenses expiring, improving performance and stability, bug fixes, and even things like widescreen support. (Original GTA San Andreas specifically looks amazing with widescreen support and some other mods throw in)

My favorite mod currently is the GTA IV downgrader, found on GTAForums, it downgrades the version of your game, making it compatible with all of the most important utilities and mods made by the community.

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.