Question

Why does air have the least permittivity?

Answer 1

permittivity itself a measure of the resistance offered by a media to the electric field.
It is the measure of capacitance that is encountered when forming an electric field in a particular medium.
In the clear sense the energy of the field is stored in a media by distortion(migration, polarization, reorientation, etc) of electric charges present there. If we think about free space there is nothing to distort, hence no resistance to electric field and permittivity of free space(vaccum) should be zero.

Similarly less ability shown by air& hence least permitivity