The sharpness of your shadows is going to depend on the size of your light source and distance from the object. Changing either of these may require a change to the strength of the light.
Given this you might think a sun lamp would give you the sharpest shadows, since it is infinitely far away, but it appears the sun lamp tries to do some atmospheric scattering OR it is not of size 0.

A point light will give you the sharpest shadow since it is emitting light from a single point in the scene.

Area lights can be dialed in by scaling them, but again you'll probably want to adjust the lighting strength afterward.

The same is true for emissive planes, but emission scatters light in all directions. Area lights are directional, and can be adjusted further with the new Spread setting.

