Let's look at the bare minimum amount of monitoring every business critical website needs. This is the absolutely least amount of monitoring you should have in place.
You need to make sure the site is up and available so you need at least some sort of "ping" test to the site. Does the site return a 200? If the site doesn't return a 200 you need to send an alert to support so they know the site is down. Pingdom, App Insights, New Relic, etc... all offer basic "ping" tests.
The other two critical things to monitor are cpu usage and ram. I typically setup an alert if the cpu usage is above 90% for 5 minutes. Also, I setup an alert if ram usage > 90% for 5 minutes. You can tweak these as needed.
In summary the bare minimum monitoring you should have in place is the following:
- Ping monitoring a.k.a. availability or uptime monitoring
- Cpu monitoring - Alert if cpu percentage > 90% for 5 minutes
- Ram usage monitoring - Alert if ram usage > 90% for 5 minutes