In a recent Q&A session (Answers to 43 Questions About Search, Social, Content, Conversions and More | SEOmoz), Rand FISHKIN (Rand’s Blog) referring to:
@randfish any opinion on which SEO plugin for wordpress is “best”? (IF there is good content and if not)
— Aly & John (@hopandjaunt) January 24, 2012
I’m a longtime fan of Yoast’s WordPress plugins. They’re powerful, flexible and nearly easy enough for beginners (at least, with a little light reading). He also keeps them updated regularly and allows for some of the cool, new functionality like rel=author (to be fair, you can do this without the plugin, too).
2015.04.25 edit: according with measurements made with P3 (Plugin Performance Profiler) WordPress Plugin, WordPress SEO Plugin requires lots of resources.
Note: Also see the Yahoo! Group on which I present similar issues: IMRo. To join, email firstname.lastname@example.org and reply to the confirmation email.