Depends - if you just need data storage that's fast, and don't care about having 100% data integrity, Mysql seems popular. I wouldn't trust people's money to it, though.
I've long preferred PostgreSQL, because it's always had those things that make a relationanl DB a relational DB - foreign keys, lots of consistency checks, things like that. I have to admit, though, that as of Mysql 5, with InnoDB, it has started to resemble a "real database".
Of course, for some applications, data integrity really doesn't matter that much, and you'd prefer speed, so Mysql with its native db type might be faster.
I've long preferred PostgreSQL, because it's always had those things that make a relationanl DB a relational DB - foreign keys, lots of consistency checks, things like that. I have to admit, though, that as of Mysql 5, with InnoDB, it has started to resemble a "real database".
Of course, for some applications, data integrity really doesn't matter that much, and you'd prefer speed, so Mysql with its native db type might be faster.