What type of dining is the best?
It's up to you. What is best for me many not work for you. Assigned seating puts you at the same table with the same people and waiters for the whole cruise. This is preferred by many of us since you meet new people and share your cruise experience with them each night over dinner. Having the same wait staff also is preferred by many because they learn your preferences and also often become friends.
The open seating (Personal Choice) dining does give you control over when you eat since you are not tied to a specific time to be there. If you like a particular waiter you can request to be seated in his/her area but there is no guarantee. And you can choose to eat with just your group and not have to share a table.