A word of mine. Before signing an agreement, know the strategies they follow. SEO is completely changed now. All the methods that were implemented in 2000's are not working today. Large diversification in link profile with huge quality links needed which is a tough task. So, ask all these factors before moving forward and if you get positive opinion on their answers, consider that firm as best.
Hope this helps.
Your Website is Like a Cake
Search Engines Are Looking For
Know Your Business Model
Don't Forget to Optimize for Multi-Channels
\Optimizing for Different Types of Results
Find on Google & Yahoo. There are lots of SEO Agencies.
Contact with me I can help you out
my skype ID is = paceseo
