ggunn:You'll likely get as many different answers as you get responses, mostly cheerleading. What kind of experience are you looking for? A big slow boat with a head and opportunities to socialize or small fast one with a small group? One that caters to advanced divers or newbies? Do you care if the op keeps your gear every day and returns it to you rinsed and dried? Do you want snacks between dives or is water enough? Do you want to have your SIT at a beach club with waiters and refreshments, or is a deserted pier fine with you? Do you need large tanks, or can you get an hour out of an AL80? Do you want to be the first one out at the site, or would you rather sleep in?
Different combinations of all the above, and more, are available. There is no "best" dive op for everyone, and that's a good thing.
Gordon, this is the point I made above in my post. My dive op of choice is Living Underwater, but I don't recommend them to everyone who asks for a "good dive operator" in Cozumel, unless they are looking for the services and price range that Living Underwater offers.
In alot of destinations you've pretty much got to pick the hotel/on-site dive shop combo that comes closest to what you like and want to spend. In Cozumel there's accommodations and dive shops for every taste and budget. It's just a matter of deciding what you want and then finding the ones that meet your needs.