Fastest way to export dataset in excel format is:
DataSet dsExport = new DataSet();
System.IO.StringWriter tw = new System.IO.StringWriter();
HTMLTextWriter hw = new HTMLTextWriter(tw);
DataGrid dg = new DataGrid();
dg.DataSource = "Dataset to be exported"
hw.WriteLine ("<b><font size=’3′>Exported report</font></b>");
dg.DataBind();
Response.ContentType = "application/vnd.ms-excel";
EnableViewState = false;
Response.Write(tw.ToString());
Response.End();