Non-Newtonian fluids are those fluids that do not conform to a direct proportionality between shear stress and shear rate, and yet no single equation has been proved to describe exactly the rheogram of all such fluids. In this study, five major non-Newtonian rheological models–-Bingham, power law, modified power law, Casson, and Robertson-Stiff–-have been adopted in order to get more options for selecting the best model that fits accurately the actual flow curve of a given non-Newtonian fluid. In this approach, it has been considered that the model which gives the lowest absolute average percent error (AAPE) between the measured and calculated shear stresses is the best one for a given non-Newtonian fluid. Selection of the best model is of great importance in obtaining correct results for pressure drop, hydraulics, and settling velocity calculations.