Although the definition of Mulliken is intelligible since it is directly related to atomic orbitals, generally the values of Pauling or Allred-Rochow are used.
As these values are not much different, the Pauling electronegativity is appropriate when choosing only one.
Electronegativity values change not only by definition, but are also considerably affected by the bonding state of atoms, and they should be used with considerable caution.