What do you mean by best?

What features are you looking for?

What is your comfortable tech expertise to configure, organize, and maintain monitoring tools?

There are a few good OSS monitoring tools out there.

Alerta - Distributed, scaleable and flexible monitoring system.

Canopsis - Opensource Hypervision and Data Aggregation Software

Cacti - Web-based network monitoring and graphing tool.

Cabot - Monitoring and alerts, similar to PagerDuty.

Centreon - IT infrastructure and application monitoring for service performance.

check_mk - Collection of extensions for Nagios.

Flapjack - Monitoring notification routing & event processing system.

Icinga - Fork of Nagios.

LibreNMS - fork of Observium.

Monit - Small Open Source utility for managing and monitoring Unix systems.

Munin - Networked resource monitoring tool.

Naemon - Network monitoring tool based on the Nagios 4 core with performance enhancements and new features.

Nagios - Computer system, network and infrastructure monitoring software application.

Node-Bell - Real-time anomalies detection for periodic time series, metrics monitor.

Observium - SNMP monitoring for servers and networking devices. Runs on linux.

Opsview - Based on Nagios 4, Opsview Core is ideal for small IT and test environments.

Riemann - Flexible and fast events processor allowing complex events/metrics analysis.

Sensu - Open source monitoring framework.

Sentry - Application monitoring, event logging and aggregation.

Serverstats - A simple tool for creating graphs using rrdtool. (source on github)

Seyren - An alerting dashboard for Graphite.

Shinken - Another monitoring framework.

Xymon - Network monitoring inspired by Big Brother.

Zabbix - Enterprise-class software for monitoring of networks and applications.

Zenoss - Application, server, and network management platform based on Zope