Customer opportunity reports are accessed from the top of the customer list (the icon with the tick and three horizontal lines beside it).  This will pop up a list of opportunity reports or reports about the customers.

You can look at things like the current top customer opportunities.  Quite a sophisticated algorithm that looks at which customers and which products mix could present the biggest opportunities for your customer base. 

You can also look at the top customers by this week, this month or this year.  If you are working with a filter of customers, for example, just your particular group of customers, then those customers will be what these reports are based on, not the entire data base of your customers.