OpenNMS Horizon 24.0.0 is the latest stable release of OpenNMS.
It contains a large number of bug fixes and enhancements, most notably adding machine-learning-guided correlation of alarms, and many improvements to Netflow/IPFIX/sFlow support.
For a high-level overview of what's changed in OpenNMS 24, see What's New in OpenNMS 24.
The codename for 24.0.0 is Hal 9000.