First off, are you familiar with the Quality of Experience Power BI reports? If not, I can't recommend checking them out enough! (http://aka.ms/msqerpbi)
To answer your question, it's really going to come down to what types of issues you commonly observe with your users but I'll go ahead and make a few recommendations:
Media Quality Metrics
1. Keep an eye on your overall poor stream rates for audio, video, and screenshare: Poor Audio Rate, Poor Video Rate, Poor Freeze Rate, and Poor Sharing Rate
Media reliability Metrics
1. Drop Failure rate (target a drop failure rate of less than 3%)
2. Setup failure rate (target a setup failure rate of less than 1%)
Keeping an eye on the average and average max values for your user's stream is also important. In particular, keep an eye on the following:
1. Average and average max packet loss
2. Average and average max jitter
3. Average and average max round trip time.
Use things like the Power BI report to look slice and dice your data to get a clearer picture of where your issues are. There are User Detail reports that allow you to filter into one specific user and look at different metrics that could explain that user's Teams experience.
If you don't have a CQD Building File uploaded to your tenant, consider doing that as well. By adding the metadata about your network locations to your tenant will give you enhanced capabilities to slice and dice by building name, network name, or city/state/country.