The are many threads in the "Florida Conch Divers" section on the keys. If you can post your experience level (new diver, advanced diver, etc) that can help. In Key Largo, the Spiegel Grove is a big attraction, but is an advanced dive. The majority of the reefs are in the Key Largo/Tavernier area, but further south - Marathon and Looe Key also have some nice reefs and wrecks also.
Where are you staying? Do you prefer a large boat or small boat? Many people on the board can give you advice on the FL keys, but it would help to have a little more information as to your experience and where you plan to stay.