Email reporting - guide to and best practise

I would start with.....what's important to the business? Try and get either that metric, or leading indicators of it. For example, if you run a SaaS company, then maybe your key number in marketing is number of free trials. 

You can then identify the emails / programs that tell that story. Personally I only use Email Performance Reports and Email Link Performance Reports, but that's because we do most of our reporting in Looker, our BI tool. 

You can throw filters over those reports by clicking "Smart List", and get them sent to users' inboxes using the scheduling tool. 

I hope that helps! 

Couple more hints to report types I find helpful:

Engagement Program Performance - only if you're running Engagement Programs, of course. But it's a decent, easy to set up report, which I would encourage everyone to set up for each EP you're running. Have it sent to you as often as the EP fires, so you can always see at one glance if all emails in your program are actually sent.

Campaign Activity - often overlooked, but immensely helpful if you have a ton of batch campaigns that do certain things outside of email sending. Subscribe to them to be alerted as soon as they start to process an unreasonable number of records.

Lead Reports with Custom Columns - very powerful, but cumbersome to set up and also quick to overload in a way that they hardly process at all. But a lot of interesting answers can be given by this report type. Definitely worth diving into!

One general tip: Don't let the number of reports you subscribe to get out of hand. If you start disregarding them because you subscribed to too many, you have broken the system. Be diligent about unsubscribing from and disabling no longer used reports. Reports only make sense if you mentally process them.