What is the best WordPress permalink structure?

The default WordPress permalink structure is not very SEO friendly as they looks like some “domain.com/?p=123” URL format.

From the SEO point of view most likely the “domain.com/postname” alone or the “domain.com/category/postname” URL formats are the best choice.
/%category%/%postname%/

I recently use the “Pretty Permalinks” option which gives the “domain.com/2008/05/16/postname” URL format. I don’t change my site permalink format, because i started the blog with this option for better readability.