From benchmarks and experience, the best two performing databases have been SQL Server and Oracle, but i once migrated to an Oracle database and it choked hard on uncorrelated sub-queries (surely fixed by now).

Relative to MySQL, I feel like I can handle 10x the amount of data in a query, and I never have to worry about the order of my joins, or the number of tables.

For simple apps that join few tables, and dealing with 10's of k records, the difference between databases is negligible. Dealing with 100 K records and multiple tables, MySQL degrades where SQL Server doesn't really degrade till about 1 M records.