After the publication of key works regarding hexagonal and triangular cores for lightweight mirror structures, optimal core shapes had become a closed issue. Since then, the mirror industry believed that the hexagonal core is the stiffest. This conclusion is found to be a result of misinterpretation of experimental data and an unfair comparison between mirrors with hexagonal and triangular cells. The major deficiencies in previously published work, presented here, along with our additional analysis, show that the reverse is true. To clarify the details, formulas for shear, torsion, and bending deflelctions will be derived for unit cells of different core structures. In addition, finite element models will be constructed to further support the claim.