Every photographer is different, so each needs to decide for themself which agencies are best for their needs. Because many agencies require new photographers to sign a three year to twenty year exclusive commitment, you must examine any agency very carefully. Review financial records if you can, talk with some of the photographers the agency currently represents, and read the fine print in their contract - don't rush into signing anything.
Two excellent books that list information about specific stock photography agencies and their specialties are the American Society of Media Photographers' Stock Photography Handbook and the Picture Agency Council Of America Membership Directory. The STOCKPHOTO A-Z Worldwide Agency & Library Listing can also be used to locate many photographic libraries.