I'm not an expert, but I use this one to edit. W3 Edit by Sarcen : An advanced decoder capable of altering some files otherwise not accessible. Mod Kitchen : an useful GUI to use the Mod Kit above. Official Mod Kit : irrelevant of how much more we would like the official mod tools to be, the mod Kit is a must when it comes to mod The Witcher 3. I expect the textures & meshes genius have tools they use. By all means, please, help me make that list as exhaustive as possible. This list aims to be exhaustive, but surely won't be right away. So, I thought this morning, why not make a thread ?
I often found myself in a position where I was in difficulty to find which tool I need to do something. Modding The Witcher 3 : a collection of tools you need