We use Flyway to migrate Database schemas to a newer version, this allows us to update the Database without forcing completely breaking changes. For Database Version that are being supported by Flyway check out this page.
The official Ree6 bot is using MariaDB and it is our main testing suit, so we recommend using MariaDB.
SQLite allows you to have a single file as your Database, we sadly currently do not recommend using it because of some Issues related to Flyway.
H2 is similar to SQLite but allows the creating of an embedded Server to allow multiple Applications to access the same Database.
Postgre is a simple and efficient Database solution it's not like H2 or SQLite but rather like MariaDB. If using MariaDB is not an option for you, PostgreSQL is the way to go.