from Jason Belzer of Forbes,

When it comes to successful sports agents, no one can say "show me the money" louder than mega agent Scott Boras. Representing over a hundred of Major League Baseball's top players in a salary cap free system, Boras has negotiated over $1.6 billion in current contracts with estimated earnable commissions over $83 million. Not only has he carried himself to the very top of the sports agent business, but has also made Boras Corporation the second most valuable sports agency in the world, even among those agencies that represent all other major team sports combined....

Our top 10 list includes a equal distribution of agents specializing in each sports of baseball, basketball and football (3 of each) but only a single hockey agent (#10 J.P. Barry of CAA). The lack of hockey agents is most probably caused by the tendency for all-star players in the league to be locked into long-term, lower salary deals, and those players signing with only a few larger, more experienced agencies.

more on the Top 10 sports agents including this on Barry...

Current Contracts Negotiated: $547.85 million

Maximum Commissions: $21.91 million

Most Valuable Clients: Sidney Crosby, Daniel Briere, Anze Kopitar, Dany Heatley, Evgeni Malkin