Final answer:
The Server Farm is the most powerful entity in the Computer Hierarchy list provided, due to its ability to handle large-scale tasks with numerous interconnected servers.
Step-by-step explanation:
The member of the Computer Hierarchy that is the strongest/fastest in terms of its relative processing power among the options provided is a Server Farm. A server farm consists of a large number of networked servers, which are used to provide services such as data storage, processing power, and hosting applications. This type of infrastructure is designed to handle massive amounts of data and provide high computing power and resources for complex tasks. As such, a Server Farm is more powerful than a Personal Digital Assistant, Personal Computer, or even a Mainframe when it comes to raw processing capability.