Various ways of generalizing the concept of maximum overlap or Brueckner orbitals (BOs), which are defined in single-reference (SR) state formulations of many-electron theory, to the case of multi-reference (MR) state approaches are proposed. These generalizations can be classified into two categories. The first one consists of orbitals that yield the maximum proximity of the model space, , spanned by d-determinants constructed from the orbitals considered and the target space, , spanned by the set of d exact wavefunctions of interest in the MR approach. Due to the fact that there are many proximity criteria, the first category includes various maximum proximity orbitals (MPOs). The second category includes the set of orbitals (MR-BOs) which satisfy the generalized Brueckner condition (the requirement is that in the configuration-interaction expansion of the exact wavefunctions in the intermediate normalization there are no singly-excited configurations). Interesting relationships between various MPO-type sets, as well as between MPO and MR-BOs sets, have been disclosed. It has been shown that there exists such a proximity measure of and that the orbitals maximizing it simultaneously satisfy the generalized Brueckner condition. These orbitals seem to provide the most satisfactory MR generalization of the BOs. To illustrate the detailed structure of the various orbital sets considered and to test the sensitivity of various proximity measures results of calculations for the H4 model are presented and discussed.