Basically Ado.Net supported all type of database( data providers) but most of the developer use Sql server(System.Data.Sqlclient) for communicate with data

There is no default provider. Basically ADO.net supports 6 type of providers, and to use each one of them you have to incorporate the assembly or namespace in the code. So there is no default provider as such. For further reference you can read this link- http://msdn.microsoft.com/en-us/library/a6cd7c08.aspx