Question: What is the best and commonly used datasource for an iOS Application?
I am a .NET developer and want to use mssql server for database. Any suggestion on that? webservices, xml providers, etc...

Answer: 
Just about every mobile app that deals with databases uses [SQLite](https://www.sqlite.org/). There are many tutorials on it.
The CoreData Framework, which is the iOS DeFacto DataStore utilises SQLite. Lot's of support and tutorials on it online.