I would say the most important directory to be listed to is the DMOZ directory.
http://dmoz.com/
It is currently, I believe, the largest and best known directory out there. It has a high Pagerank and is constantly referred to by search engines. It is all human-editted, so submission also takes a nice amount of time. If you are to submit there, don't submit again, be patient, as some of the editors there are evil.
