The thermal system design strongly depends on material selection. Nanofluids offer design flexibility and finetuning of properties by incorporation of nanoparticles in base fluids. This flexibility is guided by particle-particle communication, which may be beneficial in creating ballistic routes in heat transfer but also detrimental due to affecting nanofluid properties. The transition of nanofluids to industrial use requires application-based examinations. For this purpose, different types of nanofluids were investigated in this work in terms of their thermal efficiency in a flat plate solar collector (FPSC) and some figure-of-merits (FOMs), under laminar and turbulent flow conditions. Investigation of both aims at clarifying the correlation between FOMs and FPSC thermal efficiency, and further reporting on the validity of FOMs in assessing thermal efficiency. Results indicate that nanofluids' eligibility as a heat transfer fluid depends on the flow condition, since a base fluid could outperform a nanofluid under turbulent flow. Nanofluid type and nanoparticle shape affects thermal performance, as suspensions of nanoplatelets/nanotubes in low concentrations ( < 0.04 vol%/0.25 vol%) are shown to outperform certain spherical metal-oxide nanoparticles ( < 3 vol%), according to some FOMs. It is shown that performance evaluation criteria (PEC), overall energetic efficiency, and energy ratio (ER) do not capture FPSC thermal efficiency trends, e.g., for graphene nanoplatelet nanofluid, as Mouromtseff number-based comparisons do for laminar and turbulent conditions. It must be highlighted that the FOM type to indicate thermal efficiency should be chosen depending on the application, and simultaneous consideration of thermal and hydrodynamic characteristics is required.