The best way to retrieve large amounts of data from a SQL Server (or SQL Azure) database

I have a table with 40 millions rows. I'm looking for the best technology for retrieving this data to JSON via API.

The performance is critical!

I think, the best way is to use ADO.NET and a SqlDataReader. But, I want to use automapping (like EF).

Could you tell me that is the best way to retrieve large amounts of data? (2, 3,.. 40 million rows)