Web Directories are considered very low value compared to what they was considered 10 years ago. While it is a normally a good idea to have a few its actually bad to have all web directories.
Google doesn't want Webmasters making links to manipulate the results, Google has a strong stance on this topic and considers link building by webmasters punishable by many updates they do regularly. Not to say 15 web directories will get you punished it won't but if you had 100-1000 without any linking platforms then I'd expect to see your page on 210, never-mind 21.
Web Directories are low value because they are considered to be THIN content, Don't offer a experience for people visiting them and the information mostly found on them are copied and pasted from the sites they are linking too, never-mind the fact that these sites link out to 10,000's of site's they pass little JUICE.
Going back to what Google wants, they want Natural Links made by Your Audience. Now you could have a website that is a zillion times better than the ranking number 1 website in the niche that your focusing on... But Google values your content and the links, the chances are number 1 has more links, age and the content... Age is one of the hardest factors to beat. Afterall, if you was running this site for 10 years would you want people coming along and knocking you off the top spot with ease? Of course not. You need to EARN number 1, and in fact even the FRONT page is good depending on the search volume.
Linking Diversity - It Matter's
Google works on linking diversity, what this means is that if you have 15 links in web directory and non on any other platforms such as Blogs, Wiki's, Forums, Social media then those 15 links are devalued even more - because its very obvious that you created those yourselves.
It's like if I was to build 1,000 blog comments and have 0 on forums, social media what are the chances of having 1,000 blog link comments without the other interactions? None, it won't happen and they will come down hard on my site for doing so.
The best way to get people to interact with your site and link naturally is to use SOCIAL media, make a twitter account, use pinintrest, facebook, instragram, and so on.. The list goes on.....
Personally the best way of getting link is becoming a PRO in the field of your niche, Join forums become valued and they will let you link to their site within your posts. Blog on other peoples sites using your knownledge and have author links to your site. Times have changed, it's no longer about how many links you can get, its about quality of links and getting to know people so you can get those links.
Furthermore, Page Rank doesn't mean much. I operate a few PR 7's and while they are found easy on some searches others they are not. PR doesn't mean your site will rank, ignore it. What you want is REVALANCEY.. IE if your site is about CATS then its best to have link from another site that is purely about CATS.. If you get a link from a site about Pets... Then while its good as cats are pets and they will have pages about cats it won't be powerful as a PURE cat site. If you get a link to your CAT site from a CAR site, or web directory with plumbing, it services, web design and so on.. Little rankings are passed to help you.. After all, its a junk like.
Focus on quality content as your doing and focus on engaging people on forums, blogs, social media.