The 4-ohm speaker would be the loudest. The on-board op-amp is only a fraction of a watt, so it is not loud by itself. But you can connect it to an external amp for more volume. We have this class-D amp: http://www.adafruit.com/products/987
But it will also work with any powered speaker systems.
Computer speakers are great for stationary use. If you need to go portable, there are speakers designed for iPhones and MP3 players that will work with the shield as well.