Seach engine optimization company is quite a big word for me. When you say firm or company you mean a group of people ( include here the manager ) that work on SEO for some time and this is the only thing they do all day long ( even if some of them have desing or programming experience also ). Except that you should have in such company a small group of people able to do the link building part , again you will need a project manager , a person that knows well what he does.
My question is : DO you know the inside of a SEO company or Firm so that you can say if a company or a firm is 'the best' ?
Of course there can be smaller firms but not totally pointed for SEO , for example some guy is a programmer , some is designer , 3 guys do SEO , 1 guy is a SE consultant and so on...
PS: another 'of course' ...this is just my opinion
