Efficient way to get all the field (column) values in an array from SharePoint List

Even I will suggest using DataTable than array

SPWeb oWebsite = SPContext.Current.Web;
SPList oList = oWebsite.Lists["List_Name"];
SPListItemCollection collListItems = oList.Items;

//DataTable oDataTable = collListItems.GetDataTable();

DataGrid1.DataSource = collListItems.GetDataTable();
DataGrid1.DataBind();

you can either use grid or the commented line if want in datatable format for further functionality.

Or

You can get list items collection in a generic list.

List<SPListItem> listItems = list.GetItems(query).Cast<SPListItem>().ToList();