system monitoring and the three bears

DISCLAIMER: I work for the company that makes the software I'm about to suggest. Despite this, my opinions below are entirely my own and in no way representative of those of my employer.

My personal favorite is Netdata.

Pros:

• It's dead simple to install and set up. Most cases require little to no manual configuration at all.
• When you do need to configure it, the configuration is reasonably easy to understand.
• It comes with it's own built-in data storage, unlike some other options which require you to provide your own.
• It comes with it's own built-in web dashboard for viewing the collected data, unlike some other options which require you to provide your own.
• It has very low overhead for what it does. As a sample, on my home server, I'm seeing an average of less than 1% CPU usage and less than 256MB of RAM usage to collect just short of 4000 metrics at per-second resolution. Most of the RAM usage is actually configurable in-memory caching of the data too.
• It's got easy integrations for a wide variety of alerting mechanisms and easy configuration for automated alarms.
• Writing new collectors is dead simple. The interface between data collectors and the main process is a reasonably easy to understand and implement plain-text protocol over stdin/stdout of the collector itself.

Cons:

• Support on platforms other than Linux is somewhat limited (right now). It runs fine on FreeBSD (mostly) and macOS (with limited reporting) but not really on anything else. Windows support is planned but probably quite a ways off, and other UNIX-like platforms are pretty much not supported unless they look from userspace sufficiently like Linux/FreeBSD.
• The library of data collectors is somewhat limited compared to many other offerings (though it's improving over time).
• Centralized data collection functionality is somewhat limited right now. You can stream from multiple systems to one central system, but it's a pain to get aggregate data from that main system. There's a cloud-based option to remedy this in the works though.
• Native binary packages are somewhat lacking on many distributions (or they're horribly out of date).