I've voted MySQL because it's the one I personally choose where possible, but applications I write interact with various kinds of datastore including Oracle and

I've voted MySQL because it's the one I personally choose where possible, but applications I write interact with various kinds of datastore including Oracle and Microsoft SQL Server. (We probably have more of those.) One application I made has its canonical datastore inside a series of SharePoint lists, which gets sent to me in CSV via Microsoft Access. Well-meaning but technically incompetent users will find all sorts of ways to store their data!