[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SetWindowPos(IntPtr hWnd, int hWndInsertAfter, int x, int y, int Width, int Height, int flags);
/// <summary>
/// 得到当前活动的窗口
/// </summary>
/// <returns></returns>
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern System.IntPtr GetForegroundWindow();

哪个窗体想要置顶，在Form_Load中加上

SetWindowPos(this.Handle, -1, 0, 0, 0, 0, 1 | 2); //最后参数也有用1 | 4　
具体说明，看API函数说明
如果是用点击一个按钮后弹出新窗体，并置顶，则：

Form2 frm = new Form2();
frm.Show();
SetWindowPos(GetForegroundWindow(), -1, 0, 0, 0, 0, 1 | 2);