Instead of defining HDR=No why not let the driver to get the column names (Hdr=YEs). Using this the column names in your datatable should correspond the ones in Excel.
What comes to the maximum values, after filling the datatable, selct the max using a
dataview [
^]. Something like
...
DataTable AllData = new DataTable();
da.SelectCommand = cmd;
da.Fill(AllData);
AllData.DefaultView.Sort = "ColumnName DESC"
...
For more info, see
DataView.Sort[
^]