In Playa I dived with Scuba Libre (basic recreational diving) and Protec (more advanced cenote & cave diving) and in their different ways I'd recommend them both. Can't speak for the ones I haven't used, of course.
I thought the ocean diving off Playa was quite inferior to the diving I'm used to down here in Belize.
I did dive with "Dive Paradise" out of Cozumel, and as I was in a group of four instructors (and a dive shop proprietor myself) I thought we'd get the best they had to offer. But far from it, the dives we did were IMO lousy, and although we'd pre-paid for four dives we all decided to skip the second day. So I still have a voucher for the untaken dives (they wouldn't refund the money paid). Can't recommend them, and it pains me to say that about another operator, but I cannot accept charging someone for a service that for whatever reason isn't provided.
On the Divemex question, I don't believe straight advertising is what this board is intended for....? They may be very good (I don't know them) but IMO it should be a customer that says that.
Just realised there may be some doubt about my independence - Protec is in no way associated with my company Protech and the similarity in name is purely coincidental. But they ARE the leading tech diving operator/trainer in the region.