Most general propagator in quantum field theory

One of the most important mathematical tools necessary for Quantum Field Theory calculations is the field propagator. Applications are always done in terms of plane waves and although this has furnished many magnificent results, one may still be allowed to wonder what is the form of the most general propagator that can be written. In the present paper we find the most general propagator in the case of vectors and spinors, and in this case whether they are singular or regular, by exploiting what is known to be their polar forms.