Is there a best agency
There has to be a right fit and the fit whilst being on a multi level of reasons the bottom line comes down to costs. What are your budgets. Who can you afford. There are some services that are standard so price is standardised. Eg, Facebook marketing could be a percentage of your ad spend to manage the campaigns.
The reason for having a pricing structure like this suits both parties. Mainly because it is easy to understand. Plus it focuses on your other activities. If your business is not at the stage of growth you will be looking at ways to copy how the agencies setup marketing campaigns and do it yourself.