I`ll think you`ll get a lot of recommendations and perhaps it is useful to explain more detailled what is important for you. (Yes, you explained it, but perhaps more detailled is useful). Than you have more detailed criteria to fulfil for a service and the people here can give you recommendations which match your criteria better.
high quality content in first tier, lots of diversity in links and anchor texts, multiple tiers, monthly subscription,...
What I mean is, I know some services here which fulfil all this criteria commonly. So you`ll get a lot of recommendations.
The differences are in the details. And there`re also different interpretations about much things.
"High quality content" can be a hand written article (about your sites/niche content), a hand written article about something else with your keywords in it, for other people a very good spun article 90% unique is "high quality", for other is "spun and readable" "high quality".
Link diversity, anchor etc. is the same.
And you can use a service which matches all your criteria (and makes hand written articles etc.), but makes all/a lot of links with same usernames, same themes, or other patterns which leave footprints. (this is one example for what is important for me, no patterns, no footprints, doing the work with high attention for this details. And those services are more expensive.).
For me this is important. Other people think "google doesn`t care, this details aren`t important". ( I don`t think or say " my opinion is right", I don`t know this, it is only my believe and experience).
So this is an example for "it depends on "what you need/want in detail".
Where the links come from is the same, there`re also so much differences.
So my first recommendation is to read more (here are some great threads, I`ve learned very much from them, Kimis threads are a "must read", but also others).
And than decide what is important for you and what you need/want more detailled.
Than there`re more exactly recommendations possible.
My personal experiences here (only a few, I don`t have so much money) are that some services match your criteria. Here`re some good, safe and quality services.
I can recommend kimi, because of his knowledge and experience and because of a service with very much attention to the details. (I explained before).
I can also recommend Matija (immortal Seo) because of also a very quality service and because of "individual service", he`s able to write articles/content in foreign languages (important for me).
