Yoast hands down is the best. If you are using New Rainmaker by Copyblogger their native one is powerful and super easy to use.
Ultimately engaging content that answers your ideal buyer' personas questions will be the best SEO tool to attract visitors that convert to customers.
Yes, Yoast is the best plugin for seo.
Here is a video on how to use it:
Yoast SEO is highly recommended. However, it is important that you tread lightly with regards to keyword optimization on posts.
Google does not like over-optimization of content.
In addition, keyword variations are highly recommended as you write and many built-in SEO plugins are unable to account for the benefits of keyword variation currently.
If you need help selecting more plugins for your WordPress site, give me a call.