I'm willing to bet you can find someone to swear by pretty much every resort and dive op on the island.... It's a Mecca for divers, so finding a good dive op is about as easy as dropping a rock on your own foot.
The Sabor is very nice, if you plan to stay at the resort when you're not diving. It's way south, so the taxi rides add up if you spend a lot of time in town.
Hotel Cozumel is another excellent choice, if you prefer to stay closer to downtown.
Many people will scoff at the idea of an AI on Cozumel, touting the many fine local eateries. They're right. There are tons of great places to eat. Personally, we still like the AI places for the convenience, even though we still go out a lot.
The resorts pretty much all have in-house dive ops, and I suspect you'll be happy diving with them. But for us, we dive with Aldora Divers. Fast boats, big (steel HP120) tanks, excellent staff, pick up at your resorts pier, boat dives on the east side if the port happens to be closed, and (brand new!) their own fill station, with 'guaranteed clean air or your dive is free' air and nitrox fills.
If your schedule fits, you ought to look into the SB Invasion. I think that would be a safe bet for good diving and great fun.
