Most of the SolarWinds are separated out, whereas Zabbix includes templates and capabilities for all of them out of the box. Other solutions listed include most or all of them to varying degrees as well.
New Relic is more for Application Monitoring, but the
New Relic Infrastructure is a direct competitor. Datadog and Zenoss Cloud are similar. In all cases, infrastructure monitoring is both stronger and cheaper using Zabbix. It is also available on-prem, whereas these other options are not. However, the application side is better for
New Relic and Datadog. Have not used Zenoss Cloud to determine it's strength in Application monitoring.
For SolarWinds (all components), WhatsUpGold, and
PRTG Network Monitor, Zabbix is an equal competitor. It is cheaper up front, with no recurring subscription or renewal costs (though there are support costs if you choose to purchase it). It can be more difficult, especially compared to WhatsUpGold, which is the easiest to use of all of them (and least flexible). Overall, it is a question of where your money is spent moreso than how much with any of these options.
Nagios is FOSS software as well, but much more difficult to get to a usable state. Once in place, it is probably equal to Zabbix for maintaining.
Finally, Dynatrace is by far the best solution, but it is at a significant price over and above the other options available. It was just too expensive for us to consider our needs.