Which SEO agency do you recommend?

Matt's answer:
CUTTS: A question from West Virginia, right next to my home state Kentucky, Frank asks, “If I’m looking to hire an SEO agency, which one do you recommend?” I’m not going to say which SEO agency I personally would recommend because you never know things can change, you know, people change their policies. You never know exactly what different agencies might be doing. I’ll give you a general answer to the question. If you search for, you know, Google SEO guidelines or something like that, we have an entire page on our Webmaster Help Center that sort of says, “Here are some things to look out for–and here are some things to look for in an SEO.” So, things like references, they should be able to tell you exactly what they’re going to do. Like, if they’re going to wave some magic smoke and not tell you what they’re doing, be a little worried about that. You might find what they say boring, but they should be very transparent about what they’re doing. “Oh, I’m going to change the site architecture. I’m going to do all this.” They should be completely up front about all those sorts of things. We’ve changed our SEO guidelines. So, it used to be a little more controversial. Now, they are much more focused on, you know, “Here are some great ways to identify them.” So it is a really good page and I’d recommend that you should check it out. There are lots of great SEOs out there. If an SEO gives you a sort of a queasy feeling or they seem a little shady or fly by night, there are lots of SEOs out there, and so don’t go with anybody where you’re worried that, you know, you can’t leave them or that you just get a bad feeling from them because there are lots of good SEOs out there.
by Matt Cutts - Google's Head of Search Quality Team