One of the most efficient restaurant designs to keep your customers happy is an island. This helps reduce the time between your waiters trips to the cook and the customer. There are a few different types of Restaurant City island designs, and based around the amount of cooks you have at it and the amount of tables for customers to sit at.
Making islands is a good idea in order to keep your popularity up. It keeps your customers happy and reduces them from getting up and leaving the restaurant without eating.