My usual mantra: decide what kind of diving you want to do, and you'll find a few excellent dive operations to cater to that, especially in Cozumel. Most people wouldn't know a good dive operation if it swam up and hit them in the face, and really beware of people saying this one or that one is the best or worst, since unless they dove (dived? have diven?) with every dive operation in a particular location in every type of condition and under every circumstance, the person really has no frame of reference to compare. Usually, the person went on their first dive trip and drank a lot of tequilla the last night and so that means that dive operation is the best (or worst)...in other words, believe half of what you see and none of what you hear...
CN