You can stack hexagons, but not octagons, or pentagons(?) in 2D.
( Stackable:~ I am able to arrange hexagons together without any area left between two hexagons)
So what property of a hexagon (in 2D) makes this possible?
How do I get the/all stackable shape(s) in n-dimensions?
This is an area of much research. A shape which can fill up the plane with no gaps is said to tesselate, or tile, the plane. There are an infinite number of shapes which do this in any dimension. To see this, imagine taking a square, and cutting a shape out of one of its sides. Now attach it to the opposite side in the same orientation. Two of these shapes fit together. You can tile the entire plane. For regular polytopes, the n-cube works in all dimensions, and other analogues of 3D shapes are spotty in higher dimensions. For example, the triangle can tile the plane, but the tetrahedron can't tile space.