Many merchants looking to start an affiliate program wonder which
network would be the best one to start their program?

Matt Bidinger, known by the handle of I.M.O.G. on ABW has started a great
thread with a fabulous comparison chart/graph to help potential affiliate
managers to decide upon an affiliate network:

Matt's chart and the information in that thread makes a great companion source to an article written February 1, 2013 by Rick Gardiner entitled, "Choosing an Affiliate Network For Your Business:"
Choosing an Affiliate Network For Your Business

Rick Gardiner says "Creating a matrix to evaluate different affiliate networks is probably most effective" and that is exactly what Matt (I.M.O.G.) has done. Gardiner's article explains some basics for a new affiliate merchant (and/or Outsourced Program Merchant) and guides the AM/OPM with advice and more questions leading to a forward approach.
If you are an affiliate merchant who wonders which is the best network for your program, I urge you to read both Matt's thread and Rick Gardiner's article; the combination of these two is great.