6 - Rhinoceros
While true SubD tools didn't arrive until Rhino 7, Rhino 6 introduced the foundational _SubD commands. You could finally create organic, smooth shapes (like a mouse or a car bumper) using polygonal modeling techniques, then convert them to high-precision NURBS. This made Rhino 6 viable for consumer product design in a way Rhino 5 never was.
: Previously an add-on, the Grasshopper visual programming language was fully integrated into the Rhino 6 installation, enabling easier parametric design. rhinoceros 6