I got this from Infographic. I tried to follow their sources, but they were circular. I therefore cannot vouch for this list, and I know that it is missing one very modded game, so just consider this a starting point for discussion. It is, at least, more accurate than the several sites I saw that listed Skyrim as #1. They seemed to be just opinion lists.
5. Skyrim
4. Doom
3. Half-Life
2. GMOD
1. Minecraft
From what I found, Skyrim has 70,000 mods on Nexus and 28,000 mods (probably duplicates) on Steam Workshop. However, Planet Coaster has 399,939 mods on Steam Workshop. Curse Forge lists just over 100,000 Minecraft mods (the vast majority of which don't work on the current Minecraft edition), so I'm guessing that Planet Coaster is actually the most modded game of all time. But that's just a guess.
One cool note about this list is that 7 Half-Life mods became stand alone games. That's pretty impressive.
5. Skyrim
4. Doom
3. Half-Life
2. GMOD
1. Minecraft
From what I found, Skyrim has 70,000 mods on Nexus and 28,000 mods (probably duplicates) on Steam Workshop. However, Planet Coaster has 399,939 mods on Steam Workshop. Curse Forge lists just over 100,000 Minecraft mods (the vast majority of which don't work on the current Minecraft edition), so I'm guessing that Planet Coaster is actually the most modded game of all time. But that's just a guess.
One cool note about this list is that 7 Half-Life mods became stand alone games. That's pretty impressive.