I assume you are asking for best in terms of SEO/helping you rank. Really, there is no "best", just make it informative and human readable (i.e. not stuff with keywords). Don't make it too long either. That's assuming you use words (rather than date, post number, etc). It's fine to use a combination of words and date, etc. Just focus on making sure your content is good for humans and don't spend too much time on small thinks like permalink structure. Good luck.
mine is simple as %postname%. this is the best permalink structure for me specially when you want to transfer your post.
absolutely true.. i totally agree with ryan_uk..,nothing much can provide on premalink structure for a Wordpress..