I have no one to recommend, but I can tell you who not to choose:
Never go with an SEO company that promises to bring your gambling site to the top ten.
It cannot be done just like that.
Also be careful of this:
One common scam is the creation of “shadow” domains that funnel users to a site by using deceptive redirects. These shadow domains often will be owned by the SEO who claims to be working on a client’s behalf.
However, if the relationship sours, the SEO may point the domain to a different site, or even to a competitor’s domain. If that happens, the client has paid to develop a competing site owned entirely by the SEO.