Open-source SIEM suggestions for corporate environment

The last two companies I've worked for have either had a MSSP handling the SIEM or nothing at all.

I really want to expand my knowledge and skills when it comes to implementing and tuning a SIEM. Has anyone got any recommendations on any open source offerings with a good reputation? Would be more of a personal learning project, but if it does end up helping, even better.

Amongst other things, we currently have 365 Defender, Crowdstrike, and Zscaler. Around …

corporate cybersecurity environment siem