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Second, why would you limit your universe to just Commission Junction? I personally despise CJ, and won't do business with any merchant who uses CJ as their affiliate technology. There are several companies that are more reputable, more competent, and more honest. Certainly, a lot of affiliates make a lot of money from merchants who use CJ, but a lot of affiliates make a lot MORE money through other affiliate networks.
However, for general "bestness," CJ offers that information in Account Manager- just compare the EPCs and Network Earnings of the various advertisers.
They didn't say that they did limit themselves to CJ. They were asking specifically about them. And CJ is one of the top affiliate networks out there. Not having any problems with them.
The best depends on your site.
Pick merchants that match our site content or build a site designed specifically to promote a wide variety of products. Sometimes a low number of green bars or a low EPC or both can spell a huge opportunity if nobody is doing much in that particular market.