What probably causes chloroform to have the lowest heat of vaporization?

The heat of vaporization is the heat that a substance needs to pass from liquid state to gas state.

As the intermolecular forces of liquid are low the heat required to separate the molecules and reach the gas state is also low.

It is the low intermolecular forces of chloroform which causes it has a low vaporization heat.

Chloroform molecule is CHCl3, i. e an atom of C surrounded by 1 atom of H and 3 atoms of Cl.

It is the simmetry of the molecule of chloroform which conferes it its low heat of vaporization, because this symmetry causes that most of the dipoles cancel each other.