Hi ,
SAP supports SQL,DB2, Oracle,Sybase ASE and SAP HANA for SRM MDM ,Database with SAP MDM.
Which DB is most relevant to Use: Now it comes to Your Business Requirement how much the complexity of structure and volume of data I find Oracle DB most relevant with MDM since i have Mostly worked with that and not find any issue yet.
SAP Now Support Sybase and SAP HANA(Mainly for SRM MDM) form SP 12 SO if you have structure like that holds SRM MDM then you would go with HANA since it is most Faster and Oracle.
Discuss with your Business Client keeping Future Project Expansion view and you could suggest based on that.
Thanks,
Mukesh Kumawat