All the names mentioned are good, I’ve been there the last few years. I’ve dove with: It’s A Dive, Amy’s Amoray Dive Center, Conch Republic, & Rainbow Reef. I would recommend each of these. It’s A Dive & Amoray both use large pontoon boats that don’t take rough water well, but rough water is unusual in Key Largo. If the weather is bad I give the edge to Conch Republic & Rainbow Reef because of their bigger boats, otherwise just see who’s going where you want to dive & hitch a ride. It’s A Dive will cancel a trip if they can’t fill a boat, but if they do they make arrangements with Amy’s Amoray to take those who are signed up. (They are within 1/2 mile of each other.)
The Key Largo area is reported to have the greatest density of dive-shops in the world. This is good in that competition keeps them each trying to outdo each other with service. I haven’t experienced a "bad" shop or staff there & wouldn’t expect to. Go have fun!