You are always looking for the best for your company, but, as you know, finding a top SEO agency is tough. It gets even worse when you want an agencies help with something you don't know much about, like web design or SEO.
You need an agency that would work for you. They deal with all the important principles, like professionalism, timeliness, and the type of quality that matches your needs.
A professional agency would never engage in shady, or 'black hat' seo techniques. They would never discuss purchasing links, or look to trick users coming from search engine, as well as send unsolicited emails. The should look to discuss a long-term relationship, and steady gain in business, all while setting expectations.
Your agency should be hitting deadlines, and answering your questions or addressing your comments within the same work day. This means keeping a line of communication, and sharing the same timezone.
Lastly, you need to verify references as well as signals that your SEO agency is a legitimate business. Take time to understand what they do, and find out who your main point of contact will be. You should definitely have an appointed account manager as a direct line, and of course, enjoy their day job.