The most preferable for directories - .org and beloved - .com Many of the earliest directories have - .org: dmoz.org lii.org vlib.org botw.org
Considering that the point of a web directory is to provide a resource, I would recommend a .org. A .org usually signifies relevance, authority, and being of a public service.
well better go for well know ones like .org or .net. If you put others, you have the chance of losing a visitor since he might go to a .com or .net or .org counterparts.