Hi Cancun has some very good restaurants I have been 8 times happy to share my favorites by food type.
First I wll cover the steak house scene. Seems to be a huge influx the last few years in Cancun. The top 3
Harry's ( http://www.harrys.com.mx ) It a very good steak house for Cancun similar to chains in the USA like Capital Grill etc. located across from the Ritz Carlton.
Camblanche ( http://www.grupocambalache.com/cancun... ) This is an Argentiain themed steak house with a fairly large menu beyond just steak. I enjoy it while its not the best in Cancun for steak I have always enjoyed my meals here. It located at the Forum By The Sea Mall.
Puerto Madero ( www.puertomaderocancun.com ) The same chain that owns Camblanche owns this place. Again an Argentiain them but modern and sleak inside with incredible sunset views of the lagoon. Steaks and seafood are top notch. This is my personal favorite in Cancun.
Next Italian. Seems to be universal for restaurants all over the world. Cancun has some good places for Italian.
La Madonna ( www.lamadonna.com.mx ) Located in La Isla Mall very good food with a dramtic decor. More of a tourist place but the location cant be beat.
La Dolce Vita ( http://www.cancunitalianrestaurant.com ) Some people claim this is the best I didnt care for it however. I didnt dislike it I just have had better italian food in Cancun.
Cennacolo ( http://www.cenacolo.com.mx ) In my honest opinion the best italian in Cancun beside Casa Ronaldi. Located in the Kuklucan mall.
Casa Rolandi ( http://www.rolandi.com ) Locations around Cancun and surrounding area. VERY good food coming from New Endland and having endless italian restaurants (Federal Hill, Bostons North End) This rates highly.
Finally Mexican and yucatan food.
Labna ( http://www.labna.com/index.html ) Located downtown some of the best most traditional food in Cancun they have a sister restaurant owned by the same people La Habichuela ( http://www.lahabichuela.com/english/i... ) Both are excellent.
La Destileria ( http://www.ladestileria.com.mx/cancun... ) If your looking for more traditional mexican. Not bad it is a chain but its good for what it does well and has a very good tequila selection.
Thats most of the places in Cancun that I enjoy there are many many others I have eaten at (Club Grill @ the Ritz, Thai @ La Isla, MB @ Aqua) I hope they help and you have a great time in Cancun.