SheerSEO

4.5 Tacos, but I'll round up to 5 given the very quick support.

The cons? A few buggy interface bits here and here, but the pros include really easy to use keyword research and tracking, good tracking over time, backlinks inspection, great ability to dive into competitors, and some useful tools for on page SEO.

Plus when I found a bug or two, a developer got back to me quickly to acknowledge, and then again once fixed.