I like Don Fosters. While they don't have any six packs, I didn't find any of the boats to be cattle boats and the staff are generally quite helpful and friendly. I liked the fact that they have a drop tank on deep dives (I only went on West wall dives, no north wall)and they will pick you up and drop you off from Treasure Island. They do run the Stingray City boats as a combination dive/snorkel; however, on both times I have been, it have been an average of around 12 divers and 2 or 3 snorkelers.
The only knock that I have with Foster's is they don't offer nitrox. Because many of the morning wall dives are fairly deep (80' to 100' profiles) I like the extra safety factor of nitrox. Plus as I am old, out of shape and tend to drink a bit, I love it as a hang over cure as well. Their package prices are fairly reasonable (the best deal is the 2 x 2 tank for $120, unlimited shore diving) The wreck of the Cali out front of the dive shop is a fun dive; however, as there is boat traffic around, you must drag a torpedo around which is a little bit of a pain.
I do want to mention that the Devils Grotto/Eden Rock is a great shore dive. Lots of swim throughs and a little deeper than the Cali area.
If you have your own transportation, you can hop around the island and take advantage of several good cheap shore dives ($8 US for tank and weights for air) In my opinion, GC is only second to Bonaire for number of good shore dive sites.
I got a car from Budget for $23 US daily on orbitz.com and loved the freedom that it provided.
Bottom line is that unless you insist on using nitrox, then you will likely not be disappointed with Fosters.
