What are the best Free Blogging sites for powerfull Seo backlinks.

Hi!

Need some expert opinions.What are the best Free Blogging sites for powerful SEO back links in long term.Such blogging sites which get PR very soon and looks more attractive and natural for visitors.Following which i know

1. Wordpress
   
2. Blogspot
   
3. weebly
   
4. web
   
5. Tumblr

Waiting for the valuable sugestions

Regards
johnkeets

here's a nice list: http://www.vmoptions.com/web-2-0-sites-list.php