For Windows machines wanting to use a free option, Windows Defender works as good if not better than any of the other free offerings.
For Linux systems, if you are only using the repositories than come with a major distro such a Ubuntu, Debian, Fedora and not loading third party applications from lesser known places you are safe. OSSEC / WAZUH are good but due to their more complicated setups I would consider them a more advanced form of proection, but a good solution.