Depends on the application and deployment structure. My personal recommendation from performance testing I have done myself:
If you are a smaller project with a single database (not clustered, single geo-location) then PostgreSQL is a good choice and generally what we use internally in our own projects. The Strapi website runs on PG.
If you have a larger application or need a clustered database with excellent syncing capabilities in multiple geo-locations or you need very good multi-master/multi-write (for apps with many post or put queries) then MariaDB Galera cluster and ProxySQL.
(Honestly I prefer MariaDB over MySQL any day for any reason as I don’t trust/major fan of Oracle. And I prefer to support other “truly open source” applications and databases)