You will receive a flood of suggestions, but I usually rent a condo in Key Largo off FRBO.com and dive with Conch Republic just over the bridge in Tavernier. They are right on the water. No long rides through the mangroves with CRD. I could go on an on about why CRD has come to be my choice in the Keys after 26 years of diving, but I won't because I know others here will.
IMHO, Key West diving conditions are not as constant as the rest of the Keys. Do a day trip there and hit the Vandenberg.
Im sure some will disagree or have other suggestions, but we will all agree that you need to go.