As someone who used Checkmk Raw (the free edition) and stopped because it's an absolute bastard to set up, I'm going to recommend Zabbix.
I'll specify. Installing Checkmk's agent on the servers was fine, but service discovery is completely unreliable (I could never get through a full discovery before it just hung on something unknown) and the web GUI is very difficult to navigate. Unfortunately most of the documentation only covers the paid version, and community support is pretty bad.
Switching to Zabbix was almost plug-and-play by comparison.