There are some interesting tools mentioned here. But, remember that conversion to a customer is much more important than popularity. Conversion isn't something that can be looked at too well from the outside.

Your best content is always going to be on topics that exist on the customer's pathway from the discovery of a need or problem to the selection of a solution. That type of content will almost always win out over what is popular.

Example: "Differences between Contractor and Employees" -> "Salary for X position" -> "Payroll Taxes" -> "Payroll Software" -> Conversion to Customer of a Payroll Software Company