Fiesta Americana is the best cleaniest and nicest ALL INCLUSIVE imho. You can beach dive with the folks from DIVE HOUSE and the boats pick you up to take you to PLAYACAR REEF. Iberostar ALLEGRO has the nicest beach, but food and service isn't as nice as Fiesta Americana. FA just finished a major renovation a month ago, closed for a year. Nightly entertainment at the beach with dinner was nice, lots of pools, bars, swim up bar in case diving is closed down on the reefs. Beach entry is tricky but to refresh your diving by yourself and get your water wings going the place is perfect with lots of corals, fishes. I stepped up onto the dock still in my rig and server came up to me and offered me a fish taco and cold bottle of water. Swapped out my tank and did it again.
Capt Mike - not trying to pick your post apart, but need to clarify some things so that there is no misinformation and confusion.
Fiesta Americana has been closed for almost a year - under construction - reopening date still TBD - I would not book anything there until it is in fact re-opened. So I am not sure where you got your information that i opened again a month ago - but it is nowhere near re-opening yet. If it were my guess, I would say another 2 - 3 months at the earliest.
Playacar is on the mainland (Playa del Carmen) - I think you are actually referring to Palancar Reef. ALL dive ops dive this reef (there are several sections to it Palancar Gardens, Horseshoe, Bricks, Caves)and there are numerous other fantastic dives as well.
Ibersotar and the Allegro are two completely different resorts. The Iberostar is the Iberostar and Allegro is the Occidental Allegro - in between the Iberostar and Allegro is the Occidental Grand - 3 different resorts.
Just some clarifications to avoid some major confusion for a newcomer.
To the original poster = there is a dive shop here to meet everyone's preferences and desires. Asking which one is the "best" is an impossible question to answer. Not only is it highly subjective, but unless someone has dove with every shop, for multiple days under the same conditions - there is no possible way for anyone to answer that HONESTLY. There are many cheerleaders for various operations, mine included. But even I don't claim or have I ever expected or asked to be "the best." There is no ONE best shop - the key is finding the best operation for YOU based on your needs and preferences.
I recommend staying away from the all-inclusive resorts and stay as close as you can to downtown. The further south you stay, the more limited your dive site options despite the "experts" that think staying south means you're closest to the "best" sites - yes, you're closer to Palancar - but there are many other sites you will miss if staying too far south (ie: Allegro, Occidental Grand, Iberostar).
Stay closer to town and enjoy the flavor of the island
Good luck and have a wonderful trip.