After countless attempts of failed google SEO
I agree with netmeg, those guys are solid, we have many top rate people right here in this forum such as the mods and former mods are all top flight people.
Some members will even give your site a quick glance for free and tell you what they think before you invest in heavy auditing. IMO, I think you might need an impartial 3rd party to help you manage the process.
FWIW, read the blogs, tweets and facebook of the candidates you like, see what they have to say about SEO, and if you feel comfortable with what they say and their perceived reputation, give them a try.
If they have a ton of followers and engagement, that's a clue people are possibly hinged on their every word.
Time is one criteria, how long did you wait for the site to rank?
I think your niche also matters.
Back when I did it for hire, I had a client that sold shoes once and I could easily rank them for all sorts of long tails that brought in customers but the main keyword "shoes" was dominated by the likes of Zappos, etc. and wasn't budging, top 20 or 30 best we could do without resorting to risky SEO that could end up with a big fat penalty.
You didn't mention your budget as that makes a whole lot of difference in what kind of talent you can afford and who we can recommend, such as the bigger internet marketing shops that are full of talent and use all the best SEO analytic tools.
Just beware you don't get sold a bad bill of goods. Have a 3rd party review the SEO's proposal before signing off on it as I've stopped a couple of clients from being massively ripped off because it was all marketing buzzwords, meaningless nonsense designed to confuse the customer and sound more expensive with a stunning price tag and the customer didn't have a clue.
So here's my advice:
Good SEOs give you straightforward data without a bunch of marketing buzz, it's understandable and verifiable.
Get quotes and proposals before starting.
Get the quotes reviewed.
If they claim the quotes are confidential under NDA and won't allow you to get them reviewed, bells and alarms should go off that they're hiding something, find someone else.