Yoast and Attracta are two of the best I've used, they're a little bit more expensive than others but I believe it's really worth the extra cost .
If you are thinking about hiring someone from here make sure they have a bunch of useful posts in the SEO board or you will find yourself with a huge mess. Its getting a bit better but most of the stuff I see in the services section is not even SEO in the first place. Just a thought. You have to keep your hands up and your chin down around here sometimes. Good luck with your search. The above posters suggestions are sound but unless you have a huge budget you wont get too far with those folks. If you want to do some content based stuff you can track down GuruCreation( he is around here somewhere usually). He can get shit done and wont cost an arm and leg. Nigel
If you really want to go for SEO with a reliable company then for sure "market mongoose" is one of the famous brand. This company has gone across 2k clients with recurring packages of 6 months and 12 months. They do 98 % of white hat SEO with timely report. They track each and every factor for your website.