On all 3 trips to GC, (10/05, 7/06 and 3/07), I have gone with Keith & Casey of Neptune's Divers. They have two boats, so depending on weather/water conditions, you can do North, West or South dives. Since they have a maximum of 8 divers, you will get personalized service, and dive the limits of your computer. Not only are they professional and safe, but they are great folks. They also offer free pick up/dropoff at hotels/condos along Seven Mile Beach. I'd also recommend booking in advance to guarantee a spot.
Although I have not dived with him, I've also heard favorable comments about Danny at Off the Wall.
Any questions, feel free to ask.