There really isn't such a thing as 'preferred' travel agents. But with that said, some of the bigger companies, like ours, do have an occasional special offering that is not available through other companies. Also, the larger companies will have group space available on a particular cruise that others don't offer.
Best advice is to talk to an agent who is also a Cruise Specialist and let them do all the work for you to find the best deal for you and your family.
Each of the cruise lines you mentioned offer a different experience and it's important to find the right cruise based on your lifestyle, requirements, and budget. So a Cruise Specialist will be able to assist you in find the right cruise for you.
Keep in mind, what one person loves, another will hate. So just because someone loved a particular cruise, cruise line, or ship, does not necessarily mean that you will like it.
My best advice is stay away from places saying they offer the best rates as often they are discounting, which goes against the cruise line's policies and this can end up causing you more problems than it's worth.
It's like any other profession (accountant, auto mechanic, etc). Find someone who will work for you and look after your best interest. In the end, their expertise can be invaluable. Plus, they will never charge a fee for their service, so it makes sense to utilize them for free.
__________________ Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!
Pete always has a well written, informative, professional answer.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Independence of the Seas 12/29/2013 Top-notch!