There are dozens of excellent shops on the island. The hard part is finding the best shop for you. We like Aldora for their larger tanks, longer dives, multiple 6 pack boats, valet service, , cash discount, but mostly for divemasters Jorge D., and Sharief. They pick us up at the Aldora pier across from Casa Mexicana, our favorite hotel. It just works best for us. They have some stay and dive packages going now that include free Nitrox as well.
That makes Aldora the best pick for us. Tres Pelicanos and Blue Magic have a loyal following and Dive with Martin has some of the best rates on the island if buying a multi day package. There is a new player in town called "Sharkys" that is affiliated with my local shop in OK. I'm sure their service is outstanding as well and have one of the most experienced divemaster in the island.
The biggest differences in the shops will be where they leave from, valet service or not, tank size, # of boats, surface interval location, price, and surface strategy (as a group when the first hits 500. Time limit, or as buddy's when first hits 500)
All shops are similar, but when you find the combination that works best for you, then You have found "your" shop. The good news is it will take multiple trips to Coz to get it figured out.
Safe travels,
Jay