Do you mean the major aff management sites like CJ, LinkShare, Shareasale, Performics, etc? If so then i'd say CJ
I aggree with jameskon - he must be talking about the sites designed solely for the affiliates programs. I must say that it depends on the niche, programs and many other factors. Besides there are sites made only for one program... All in all this is far too general question and there cannot be a good answer for that
as others have said, your strategy should vary dramatically based on whether you're trying to push a single product versus, say, an entire inventory virtual storefront where people browse for what they want. in either case, look at success. look at what works. for a virtual storefront, look like amazon, newegg, etc. for a single product, give a review with little else on the page (so your user has nowhere else to go) with detailed information about the product and why they should buy it. again, look for successful examples and imitate. then try to improve with testing.