How to make a WPF window be on top of all other windows of my app (not system wide)?

You need to set the owner property of the window.

You can show a window via showdialog in order to block your main window, or you can show it normal and have it ontop of the owner without blocking the owner.

here is a codeexample of the codebehind part – I left out all obvious stuff:

namespace StackoverflowExample
{
  public partial class MainWindow : Window
  {
    public MainWindow()
    {
      InitializeComponent();
    }
    void NewWindowAsDialog(object sender, RoutedEventArgs e)
    {
      Window myOwnedDialog = new Window();
      myOwnedDialog.Owner = this;
      myOwnedDialog.ShowDialog();
    }
    void NormalNewWindow(object sender, RoutedEventArgs e)
    {
      Window myOwnedWindow = new Window();
      myOwnedWindow.Owner = this;
      myOwnedWindow.Show();
    }
  }
}