PostgreSQL
The concrete implementation: types, JSONB, full-text search, extensions, partitioning, VACUUM, connection management.
PostgreSQL’s type system is where much of its power hides — timestamptz, numeric, arrays, ranges, enums, JSONB — and choosing the right type is the cheapest correctness and performance decision you will make.
JSONB gives you a document store inside a relational one, full-text search gives you a credible search engine, and extensions like pgvector, PostGIS and pg_stat_statements are why "just use Postgres" is so often the right answer.
Most PostgreSQL incidents are one of four things — connection exhaustion, autovacuum falling behind, a table that needed partitioning, or replication lag — and each is visible in the statistics views before it becomes an outage.