How can I display a form that is ‘TopMost’ for only my application, but not other applications

You can do this by setting the child form’s TopMost to False and setting its Owner property to the MainForm.

[C#]
Form1 f = new Form1();
f.TopMost = false;
f.Owner = this;
f.Show();

[VB.NET]
dim f as New Form1()
f.TopMost = False
f.Owner = Me
f.Show()