PostgreSQL
Adopt
PostgreSQL is an Open Source RDBMS with a large community, great documentation, great tooling support & integrations ( AWS, DataDog), a good set of easy-to-use extensions, and with great performance for OLTP workloads & more.
Recent versions have quite powerful JSON support and other features that make it potentially a good choice even for document storage and other “less relational” / denormalized use cases.