My opinion is that the best agent depends on what stage a player is in. Obviously, the best agent for a player just coming in to the pros won't necessarily be the one with the highest profile client list. In fact, my experience is quite the contrary.
My advice would be to listen to ALL proposals with an open mind. However, if we're dealing with a player about to be drafted/signed into the professional ranks, the "best agent" is the one who has a plan which seeks to DEVELOPE a player's potential.
What I mean by that is that an agent should do his most arduous work when a player is still developing. Training and media coverage should be paramount so as to get the most out of the athlete on the field, as well as managing his image from its infancy.
These services (trainer and media exposure), while seemingly basic and simple, are offered by relatively few agents.
Look for an agent who is looking at the big picture, and who understands his role, as the player's employee, in the athletes road to the Bigs.
FM