If you haven't been here before, and wanted "one dive", I would pick either Molasses Reef in Key Largo, or Looe Key National Marine Sanctuary off of Big Pine Key.
Why, when there are so many great wrecks, big animals, etc in the Keys and in other parts of the State?
First, the combination of beautiful hard and soft corals, the variety and structure are amazing. Second, the huge variety of tropical and reef fish life, everything from barracuda, dense schooling fish, to macro oddities if you really look. Nurse sharks and morays guaranteed. Also, a pretty decent chance of a reef shark, or large grouper, or eagle ray. Third, the shallow depth (less than 40'). This means lots of light for absolutely dazzling colors. It also means long dive times. If you rent boat and go dive yourself with a 100 or 120 cf tank, you might two hours of bottom time. The wrecks, as great as they are, are deep so the dives will be short.
The good news is that, because you will be doing a boat trip, you will not be doing one but two dives, so you can get two of the best--so do a trip that goes to the Spiegel Grove wreck, and then to Molasses for the second dive. Can't do better than that!