That's like asking for the name of a good stock broker... One person's good is another's ...well not so much...
Call the Carnival booking agent, get their name and extension and stick with them. You can always call back into the main number an get a different one if you are not happy with the service of the one you are working with. Or just use Expedia.com and book through them...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007