Looking to move from an affiliate agency to internalize the process and build key relationships with publishers/influencers.
It depends on the stage of your company, your vertical, and finally, your budget.
All of the aforementioned networks have hundreds of thousands of affiliates, but finding those key affiliates who will help you move the needle is a challenge - regardless of the network. It's a long slow process, that can take months to cultivate.
CJ is the largest network, but also one of the more expensive options. Typical costs would run ~10k/year w/setup and monthly minimums. They cater to e-commerce sites doing at least 1mm+/year in sales. They offer full service management.
Linkshare is one of the larger networks, but is also relatively expensive. I'm not sure how their pricing model compares to CJ, but from clients I've talked to, it's comparable. Like CJ, they cater to sites doing 1mm+/year in sales. They offer full service management.
Shareasale is much less expensive - $650 setup fee, and $35/mo (I think) or 20% of affiliate commissions (whichever is greater). They cater to small to mid-sized online merchants (but work w/larger merchants, too). They don't offer full service management, but do offer services/products to get you in front of affiliates (e.g. targeted emails, etc.)
ClickBank is best for digital goods. Like Shareasale, it's mostly self service, but relatively inexpensive to start.
Moving from an agency to an internal team can be a good move if you have the internal resources to recruit, train, retain your affiliates.
Best of luck with your program transition!
I support Jeremy's answer above. To add: You might want to check out Impact Radius as a platform. It's not a network (well they have a marketplace - but for the sake of this example let's keep it simple) and if the impetus is bringing it in-house, I make the assumption you have the staff and expertise to do the recruiting, affiliate promotion optimization and other responsibilities of making it a worthy and profitable program.
Here if you have any questions!
I don't suggest any affiliate networks as a tool to "scale quickly" that is not the purpose of affiliate marketing or even these networks and chances are that if you don't have the relationships in place already, you will fail at this attempt.
You need to establish the relationships first... and that is not "scalable" you have to put in the work to build key influencers as brand ambassadors and then move into affiliate marketing.