TopMost = true / false

[DllImport("user32.dll")]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int
X, int Y, int cx, int cy, uint uFlags);

IntPtr HWND_BOTTOM = (IntPtr)1; 
uint SWP_NOSIZE = 0x0001; 
uint SWP_NOMOVE = 0x0002; 

private void BaseForm_Activated(object sender, EventArgs e)
{
    this.TopMost = true;
}

private void BaseForm_Deactivate(object sender, EventArgs e)
{
    //this.TopMost = false;
    SetWindowPos(this.Handle, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
}