The Barcelo resorts near Playa del Carmen are very good, reasonably priced and are all-inclusive. However, this is about 35 miles (50 kilometres) south of Cancun.
For places right in Cancun, I always go 5-star so I stay either at the Fiesta Americana Grand Coral Beach (there are two Fiesta Americanas in Cancun), or at the Le Meridian. The major advantage of the first hotel is that it is on a great swimming beach. Most of the hotels in Cancun are on the beach with giant waves (and usually quite a few rocks). However both these hotels are not all-inclusives and not cheap. I believe the Riu (which is next door to the Fiesta Americana mentioned above), is an all-inclusive, but is also not cheap.
However, all the places in Cancun have great pools, and there are tons of wonderful hotels. I have heard positive things about the Moon Palace, Cancun Palace, Ritz-Carlton and Club Med--most of which are all-inclusives--just a little ways down the (rougher) beach.
One last thing--some hotels in Cancun allow topless bathing, some don't (of the the places I listed above, some do, some don't). Depending on your preference and the preference of your guests, you might want to check this out before you book. That way you can stay at one that suits your taste.