free seo plugin is ALL-IN-ONE SEO, payed are more like SEO-Pressor, StatBlaster, and I'd recommend you the SEXY BOOKMARKS plugin for bookmarking as increases SEO value
I have looked at a lot and for sheer usability Yoast is the one. Easily set titles, meta's, keywords, 301's canonicals, exclude taxonomies, posts or catagories. Build sitemap with changes and ping the biggest search engines instantly.
I agree that SEO Pressor is a tremendous plug in for WP you should aim for 3% keyword density and 100% SEO rating in SEO Pressor but it really boils down to inetersting content that is vital.
I would say that All in One SEO is the best one that i have come across till now. Also, a good choice can be the Wordpress SEO Plugin from Yoast. It is quite useful sometimes...
I'd agree with most the others, AIO SEO. As for your comment about your traffic spiking, I'd consider using these to help your site withstand the spikes of traffic. WP Super Cache (free) - A fast caching engine that produces static html files. WP Minify (free) - This reduces the number of files served by your web server by joining Javascript and CSS files together. Hope it helps. Chris.
If your serious about ranking your blog,you cant beat seopressor.It will teach you how to set up your on page seo,sure its not free but its worth is weight in gold.
All in One SEO pack good,But they just put title description and meta tag,But this plugins are not suggest for on page seo.So for this i use A Plugins SEO Possessor .I Think this is the king of On page optimization SEO
Vibe SEO Pack is the best SEO plugin with inbuilt SEO audit, I have been using it for years now and m completely satisfied.