The right answer depends a lot on the platform you are programming for.
It just so happens that someone asked this question for a specific platform in StackOverflow about 1.5 years ago.
Whether it is MySQL, SQL Server, Oracle, PostgreSQL or some other RDBMS, you have to be very creative with the database infrastructure. If you have an open checkbook for hardware and an industrial-strength DBMS, Oracle is for you (if fact, Oracle RAC would be more desirable). If you are developing using IIS and a Microsoft environment, then it is SQL Server all the way. If you have budgetary concerns and want an Oracle look-and-feel, PostgreSQL to the rescue. If you have budgetary concerns, a vivid imagination, and want to micromanage the Storage Engine to your liking to accommodate ACID-compliance, high speed reads, and a variety of replication architectures, I would prejudicially say MySQL.
The DBMS should be the least of your worries with MMORPG. Programming issues always present bigger fish to fry. So, make your decision prudently and wisely, because whatever DBMS your choose, you have to live with it (the same way FaceBook has to live with MySQL).