Anything you can plug into the base station's audio jack will work, so it's not really a matter of which ones are best for the AirPort Express in particular, but which speakers are...well, good speakers.

In response to the above (slightly confused) answerer, AirTunes is a feature of the AirPort Express base station that basically lets you plug speakers into the base station, then wirelessly stream music from iTunes to play over those speakers, regardless of where your actual computer is. There's a program available for Macs and Windows PCs called AirFoil, which lets you stream *any* audio from your computer to the wireless speakers, instead of being limited to iTunes.