I've used Nagios and Icinga2, and I've become a huge proponent of check_mk. The documentation isn't great, but the product rocks: with very little time, you can start monitoring a slew of services (disk, hardware, logs, ntp) with almost no tweaking required. You can easily create custom checks, but you also have all the Nagios plug-ins it's compatible with. No daemon listens on the hosts being monitored. You get graphing for free (no setup time) for almost all your checks.
It uses Nagios under the hood, it's basically an automation system that generates those Nagios systems. The GUI is amazing, because it uses a plug-in so you don't have to edit files on disk to group your hosts or tweak the alerts. Those configs are snapshotted automatically at every change, and you can replicate that configuration automatically to remote servers. Download it from the upstream site instead of relying on distro package repositories.
Caveat, the documentation sucks, the GUI can be nonintuitive and it's hard to Google problems. It takes time to fully tune. Out of the box you'll probably still be impressed though.