db_datareader, db_datawriter and db_owner permissions
According to Database-Level Roles:


  Members of the db_owner fixed database role can perform all configuration and maintenance activities on the database, and can also drop the database.


A member of this Database level role can do whatever he wants on the database. This also include SELECT, INSERT, UPDATE and DELETE data.

You still have to make sure that you are not giving too much rights to a user when you make him a menber of db_owner...

You can connect with this user on your database and run this query to see effective Database level permissions using sys.fn_my_permissions:

SELECT *
FROM sys.fn_my_permissions(null, 'database')
--WHERE permission_name IN ('SELECT', 'UPDATE', 'DELETE', 'INSERT')
ORDER BY permission_name


Output with only db_owner includes following rows: 

entity_name subentity_name  permission_name
...         ...             ...
database                    DELETE
database                    INSERT
database                    SELECT
database                    UPDATE
...         ...             ...