DMOZ and Yahoo Directories are the most important directories. Getting listed there however could be pain.
Google takes a very thin line when it comes to directories. They have no mercy for link sellers and idiots. So yeah, some directories (and only some) get the thumbs up
Look for directories that have decent traffic then try to determine if their traffic comes from webmasters submitting their site or if it's from actual people surfing the directory. A good way to tell is if the directory is listed in Google for long tail terms on their pages. Those directories will be more important in Google's eyes...and will actually bring you some traffic too.
I think they simply find some more equitable in general, and not specifically for seo purposes. Probably no more than 5 major ones. The rest of us are just guppies
Shellerz has it right. There are some others which do well on a smaller scale and there are some non-US local ones which do well. Google has been considering to completely discount all directory links, including DMOZ and Yahoo for a while. I think we already see that happening to a lesser extent.
Sure, Google uses it's proprietary rating system which means some directories will rate higher than others. I don't think they have a list of favourites as such. I would imagine they would pretty much discount the myriad of clone directories that use a script and list of standard categories and put no effort into making their directory different or of value. I have been saying it since 2005, you should only start a directory if you are going to add value to the interwebs. free directory script + standard categories + standard template = waste of space
I think one of the biggest problems is category dumps on free directories. Plus side of cat dump on free dir - very easy to find the cat you wanna submit to in most cases. Downside - well the downside pretty much outweighs the plus side unfortunately. Even if using a category dump, should at least go in and delete the never used categories, and add a few unique ones to help seperate it a bit from the other free directories. Pretty much says it all when someone starts a thread asking for a category dump literally telling you they cant be bothered building the category structure or dont have time to. So in the future can they be bothered reviewing, editing, rejecting, promoting, maintaining etc.