There isn't really an optimal - it was mostly a set of accidents to do with already installed turbines and the number of generator poles in a particular design.

There isn't really an optimal - it was mostly a set of accidents to do with already installed turbines and the number of generator poles in a particular design. Lower frequencies were easier to build for early components, 50Hz is about the lowest frequency that you didn't notice flicker on a lamp. Almost all the early non-industrial use of electricity was for lighting - industry tended to generate their own power on-site using whatever equipement they happened to buy. Telsa picked 60hz apparently because it allowed him to reuse clock mechanism in test gear. He also felt that 60sec/60min so 60hz was a 'natural' solution