The Best SEO company is?

Have a look at www.seoworkers.com and what their clients say about them  https://www.seoworkers.com/testimonials.html

Good luck. :)

There are several good resources for this. While I do not believe that any one company is going to be "The Best" as there are a few extremely good companies as compared to the many companies composed primarily of an affordable workforce with only a year or two of hands on SEO experience.

The best way to answer this is to provide you with third party resources and respected industry sites that rate, or list SEOs.

Some are pay to enter sites, but keep in mind larger more successful firms will constantly pay for top memberships if there is any value to them.

 That being said;

Top SEOs
http://www.topseos.com/
A well respected industry company that lists company information, case studies, competitions, company rankings, and news.
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PromotionWorld's Top 10
http://www.promotionworld.com/top10SEOcompany.html

Another venue that rates SEO firms and offers many articles on promotion for DIYers and more skilled marketers.
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SEO Pros
http://www.seopros.org/

Another professional organization that lists reputable firms. SEO Pros is one of the older sites to offer exposure to companies all-in-one site.
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SEMPO
http://www.sempo.org/

Organization run by Search Marketing pros, created to help promote awareness of SEO and SEM. Offers several different levels of membership and seems well respected by the biggest names in the industry. If the company in question is not listed then you may want to reconsider.
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SEO Consultants
http://www.seoconsultants.com/ 

Search Engine Watch Thread
http://forums.searchenginewatch.com/showthread.php?threadid=8200

Search Engine Watch is the one of the few forums where many big name marketers may be visiting. It is all user opinion based and offers non-objective resources. A few users do plug themselves. Many of the forum users are all regulars at the Search Engine Strategies shows that promote the SEO Industry. So it can be a decent place to learn some SEO Tips.

It also reiterates my picks above as valuable resources and offers a few others that I did not mention.

I made this post to offer resources rather than try to claim any one company is the best, as there are many factors in SEO that make that hard to claim. Also as we are an SEO firm, it would be plugging our own site.  So instead here's my own plug with a few votes we received for Best SEO Company.

If you want information on selecting an SEO firm and what to look for here's a few articles:

http://www.seroundtable.com/archives/007003.html
http://www.webmasterworld.com/forum5/6373.htm
http://www.seoimage.com/marketing_optimization.html
http://www.webconfs.com/choosing-a-seo-company-article-21.php
http://www.isedb.com/db/articles/1172/0/How-to-Choose-an-SEO-Company/Page0.html

The Best SEO Company? Yourself - through education and testing SEO. If you pay someone, you may not get your money back through sales for 1 to 2 years! SEO is an ongoing thing to keep your site at the top of the engines. SEO companies could charge you thousands. There is a way to avoid paying for SEO because to become an expert SE optimist, one can read and implement free SEO literature. Start reading info at a free

While I do think it's important to self educate and the sites listed here are good resources, I also think it's a good idea to attend an SEO conference and take advantage of listening to the wide range and variety of speakers on the different topic panels to help you make a decision.

Learning from people you feel you're comfortable with or who can speak directly to what you want to learn is valuable. 

There are a lot of SEO conferences out there:

www.searchenginestrategies.com

www.searchmarketingexpo.com

www.pubcon.com