Even just looking at Modthesims and accounting for the fact that many of the mods are just new items, there's over 70k mods/items alone.
And that isn't including the Sims 3 stuff or The Sims resource.
I will agree that Skyrim probably has more significant game changing mods made.
Yeah, I was going to say if you're just counting custom content like clothes, textures, objects, etc. it's probably something like The Sims 2 or The Sims 3. Of course, we should probably distinguish between custom content (using outside mod tools) vs. user-created content (stuff created using tools provided by the game, such as Sims lots, and which I guess a lot of Minecraft stuff would fall under?), but even by that measure there's a lot of custom content for The Sims.
If we're talking about mods that actually change some aspect of the gameplay, it's probably something like Skyrim.