Not sure how popular these are nowadays, list includes few old ones:
- If you already tried Skyrim, try Morrowind or Oblivion (I think these 2 are way better than Skyrim),
- Fallout 3, New Vegas and Fallout 4 if You are already familiar with Elder Scrolls series,
- Witcher 2 has good tool called Redkit for modders, but modding community was rather small,
- Neverwinter Nights 2 is isometric 3d. I never played it but I heard it has lots of mods,
- Gothic and Risen series, especially original Gothic 1 and 2 have/had good modding community.