You do not choose your embarkation time, you choose the time you arrive at the pier. Pick the time that you wish to arrive.
In theory, if everybody picks their time and sticks to it then there should be no better or worse time to pick, because the crowds will be spread throughout the day.
Generally speaking the time with the most crowds is early, 10AM-11AM or so. After that the crowds peter out and by 1:30 you typically breeze right through the process.