It would be the Source/Medium report.
It’s the first thing when a new client gives me access to their Google Analytics.
It’s an Acquisition report, meaning it tells us how traffic is getting to our website. Access this report by clicking on ACQUISITION, then ALL TRAFFIC, then Source/Medium.
If this is the first time you’ve looked at this report, set the date range to look at the prior 3 months.
Now… what do you see? What are the primary traffic sources for this business? Are there any traffic types, such as SEO (called ‘organic’ in GA), which are absent from the traffic mix?
You can learn a lot about a website (and a business) by running this single report.
BONUS ACTIVITY: Click on one of the traffic sources, such as ‘google/organic’ for example, and add a Secondary Dimension of Landing Page to find the pages on your site that are getting the most traffic from Google Search.
Now, repeat with other traffic sources and voilà, you’re starting to figure out what makes this site tick.