Alan Zeichick, commenting on the release of Oracle DB 11G Beta ranks the major databases as:

1. Oracle
2. DB2
3. SQL Server
4. MySQL

I've never been a fan of DB2, but Oracle DB is a rock and I would never hesitate to recommend it. (This despite my involvement with Oracle's abysmal XML-based file-system (FS? FX?) ). Of course, many .NET shops use SQL Server and I've yet to experience a real problem with that. MySQL is preferred for Rails development and, again, I've never had a problem using it.