SOBI2 is the most adaptive, the fastest, the most usable and tweakable directory extension for Joomla.
I use it for a business site I have.
I love the templating system within it. Very easy to adapt and integrate it to your website. The code is valid XHTML which I have tested and run in almost all popular browsers. Overall I have to say this script impressed me, and is the best free script I have found.