Go and do a search in Google.com for keywords like: "credit cards", "Payday loans", "debt consolidation" and similar highly competitive terms. The companies that rank those website in the top-5 of the organic results are the best in the world, (at the moment).
Depends on what your definition of best is? There are many companies that will sell you fake twitter followers and tell you that your ranking for your site will go up and then there are the honest companies that will get your slow but steady results. What I have noticed is, people want fast results which is a NO NO!
There are many companies offering SEO services. A typical business person would like a company who is offering something more at competitive price and more return on his investment.
To determine the best SEO companies - first of all take a look at how long exist the company, what guaranties offers company, what methods are used. Avoid companies offering quick results - any guarantees aren't acceptable in SEO.
One way to measure a SEO company, because you cant tell. Ask the indivuals who are in charge, if they do everything in house or outsource the workload. If so, then what parts of seo specifically are being outsourced to measure quality assurance.