Whenever this question is asked, you will get a plethora of opinions. I would advise the following:
1. Book with an Earmarked Agency. These are the agencies that have high revenue with Disney, and therefore have been authorized by Disney. They also are the ones that truly have the inside track. The top three are Dreams, Magical Journeys, Small World.
2. Find an agent in one of those agencies who fits the personality you have. Not every agent and client "fit". My agent is very brief and to the point in email. She and I get along stunningly. To some she would appear to be "short" or "abrasive". I realize that she is running one of the top online companies, and personally no longer takes new clients. I do not need hand holding, and trust her emphatically. When I call her or she calls me-we can chat for hours though, she just types brief emails. Her agents are all top notch and hand picked to reflect true Disney Service. I think any of the top three agencies are similar, the agent personalities will differ.
3. Make sure you are aware of any cancel and change fee's above and beyond those imposed by Disney. I believe Mouseear Vacations charges a $25 per person cancel fee in addition to any Disney charges. This is to cover administrative costs, I have not seen this charge in relation to Dreams, Magical Journeys or Small World.
Bottomline in my opinion is to go with an agency you feel comfortable with that also has a solid reputation with both clients and Disney. Then find an agent that feels like an old friend that returns pricing AND emails within a few hours.