This really depends on what type of holiday you are looking for and who is accompanying you.
Cancun was created by the Mexican government to be THE major tourist destination, boosting the Yucatan economy. It was literally put on the map and today you can find the International airport here and all facilities one can imagine. There is an endless stretch of hotels along the beach and you can party here 24/7. For diving, Cancun is close to Isla Mujeres and MUSA, the underwater museum. Also, it is less than 15 minutes away from Isla Mujeres (which has great divespots) and not too far from Isla Holbox, where you can dive with Whale Sharks.
One of the most popular destinations is Cozumel, a small island. It is mostly focusing on divers and tourists from the many large cruise ships that dock here. Direct flight to Cozumel are mainly from the US. In about an hour by ferry you are at Playa del Carmen, also known as Playa.
Playa is becoming popular and is growing rapidly. It is more relaxed and laid back than Cancun with no high-rises. It has a lot of (all-inclusive) resorts and it is an ideal location for families. You can reach Cozumel within an hour by ferry, the Cenotes are in your backyard and the Mayan temples are a few hours away by car.
South of Playa (Southeast Yucatan) also has various nice places to stay, such as Tulum, Puerto Morelos and Akumal. There are numerous (all-inclusive) resorts between Playa and these locations. These small towns also cover the Mesoamerican Barrier Reef and with the Cenotes scattered around, there is always one nearby. They are also slightly closer to the Maya Temples.