Top affiliates are the most important affiliates for a given affiliate program. Usually, the majority of an affiliate program revenues is produced by a few top affiliates.
Each merchant has his own definition of what is a top affiliate. More often, merchants offer dedicated management and benefits to top affiliates.
From a broader perspective, top affiliates may also refer to types of websites which generate large revenues from different affiliate programs. Cash back websites, give a cause programs, coupon websites are traditionally seen as top affiliates.