Balatharan,
That would probably be Oracle. Oracle was the first relational database we supported.
With Orbeon Forms 4.3, some features were supported in MySQL and DB2 but not Oracle, but 4.4 (coming out soon) brings Oracle back on par.
MySQL support has slower search at this point.
SQL Server is not yet supported, but we are working on unifying the relational code so adding support for that (and other relational databases) will become easier soon. See also:
https://github.com/orbeon/orbeon-forms/issues/697https://github.com/orbeon/orbeon-forms/issues/966-Erik