I think the bigger and more established the blog site, the better.
In the past 3 years, I have had about 10 blogs shut down (and all the content and links with them) because either the blog folded, they changed to a pay model, or the account was deactivated for one reason or another.
Blogspot is the best in my opinion.
Next, would probably be Myspace.
You know these sites are never going to change or go anywhere.
- for search engine purpose, blogger is the most important, promote your blog will get more important links to your web site. avoid to use blogger for adsense purpose because blogger are owned by google and will banned your adsense account if you use many blogs to promote adsense
- for adsense, it is important to get wordpress free version to create adsense blogs.
- it is better to get your blog domain name (wordpress)
and use one blogger to get links for these blogs.
Its easy to get a blog account, you can try with blogger.com or wordpress.com, and if you want to post your blogs for free of cost try with ioffersearch.com. its free for all and you can effectively market your product and website by creating the blog.