Just like Dentist, Doctors, Lawyers, Teachers, or any profession people have different personalities, morals, ethics, and strategies.
I would say that you can group SEO companies into 2 big groups. Black hat companies and White hat companies. Each search engine has an algorithm of where to rank and list a website for a certain keyword search.
A Black hat company will try to figure out how to break the algorithm. There are many companies that are very good at Black hat SEO. You can expect huge jumps with Black hat SEO strategies. BUT expect to fall in rank as fast as you climb if you decide to go with a Black hat company. As so as they are caught, results will positive effects and strategies will usually be reversed. Results will usually be quick but not lasting.
A White hat company systematically will increase the rank of your site by methods that have been “approved” by search engines. White hat SEO is much slower but the effects will be long lasting. White hat SEO is much more respected in the online marketing community. Sometimes the effects of White hat SEO is not seen until 6 months after the fact.
I would highly recommend a white hat company over a black hat company 99% of the time.
Now that the groups are out of the way… let’s talk more about finding the best SEO companies!