I think a great skill to have is knowing how to make paper/cardboard /foamcore/ models (adding some clay on top helps too). It’s super cheap it’s really fast and you can have a 3D form sitting in front of you in an afternoon.
I also find I’m more expressive when working with softer materials, there’s more room to finesse designs. Working soft before going to wood or metal or plastics is important. To me it’s the equivalent of sketching before rendering.
Best tool? Always have a a few pencils Hb, B. 2b.