Wich database i personally use to some medium SaaS CRM??nosql or relational database..
Wow, not giving us much to take there. You will find many good DBMS systems available in the relational to document based to object based. A primary reason you will find a lot of is that they have pros and cons in various application situations. With the little to take I'd say make use of the one you are acquainted with unless of course there is a driving reason to make use of another.
MYSQL will be a good idea because it is free and it is attaining acceptance in enterprise also.