The best model of design expert software would be a full factorial design if resources allow for 27 runs to capture all interactions and main effects. If resources are limited, a fractional factorial design may be considered to reduce the number of runs while still estimating the main effects and some interactions.
The selection of the best model in design expert software depends on the number of factors and levels, the resources available for conducting experiments, and the research objectives. A full factorial design is comprehensive but may require more resources, while a fractional factorial design is more economical but may not capture all interactions.