I adopt /%postname%/. The less link depth, the better. If you have write a number of posts on your site, you'd better not change the per permanlink type.
/%category%/%postname% has shown great results.. although sometimes it depends on what you are trying to optimize ... if it is for products like electronics.. then this would work great if you have several different categories... ie: "...yoursite.com/samsung/new-galaxy-phone-line-better-than-iphone" or "...yoursite.com/smartphones/new-samsung-galaxy-phone-line-better-than-iphone" Just using that samsung or smartphone cat will help greatly!
For beginner blogger, /%postname% The above is the best. For advanced blogger, who knows how to put more contents under different category, the following is the best. /%category%/%postname%
/%postname%/ is best but I better use /%category%/%postname% which is good even when you apply for Google News!