As long as they are supported, they all work well. I have lots of clients using SQL Server & PostgreSQL.
The main question is who will be managing the RDBMS and what are they familar with?
You should also look at other non-GIS systems in your organization that you might possibly integrate with, if any. If your organization is already using, say, SQL Server for lots of its operations, it may be better to stick with that than bring in an additional RDBMS.