Personally I'd choose PostgresQL over MySQL or MariaDB. Or possibly even Firebird. (I've used both.) Postgres is more of a "big boy" so is suitable for installs you have control over, Firebird is better for deployment to many customers. The Clarion community is reasonable for both, so both are pretty well supported.
The licensing for MySQL is really unclear. So I'd probably pick MariaDB out of the two of them. However I don' have any experience with either.
Back in the day when I did some research MySQL was inferior in most ways (although was more popular in the general public's mind.) I'm sure it's come on since then though.
cheers
Bruce