попробуйте использовать следующий
this.FormBorderStyle = FormBorderStyle.None;
this.WindowState = FormWindowState.Maximized;
this.Bounds = Screen.PrimaryScreen.Bounds;
this.TopMost = true;
в случае PInvoke попробуйте этот код
public static IntPtr HWND_TOPMOST = (IntPtr)(-1);
public const int SWP_SHOWWINDOW = 0x0040;
[DllImport("user32.dll", CharSet = CharSet.Auto)]
public static extern int SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter,int x,int y,int cx,int cy, UInt32 uFlags);
И назовите это
IntPtr handle = this.Handle // or Handle to another window
SetWindowPos(handle, HWND_TOPMOST, 0, 0, Screen.PrimaryScreen.Bounds.Right,
Screen.PrimaryScreen.Bounds.Bottom, SWP_SHOWWINDOW);