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There are only five space-filling convex polyhedra with regular faces: the triangular prism, hexagonal prism, cube, truncated octahedron (Steinhaus 1999, pp. 185-190; Wells 1991, pp. 233-234), and gyrobifastigium (Johnson 2000).
So depends on what you mean by simplest but I would say the either the cube or the triangular prism
