The easiest way is:
Process.Start("shell:Downloads");
If you only need to get the current user's download folder path, you can use this:
I extracted it from @PacMani 's code.
// using Microsoft.Win32;
string GetDownloadFolderPath()
{
return Registry.GetValue(@"HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders", "{374DE290-123F-4565-9164-39C4925E467B}", String.Empty).ToString();
}