First, how am I defining a party resort? I am going to keep it simple - a resort with an active pool with music and activities during the day and an active scene at night. That means good entertainment that goes late into the evening. Let me explain:
The first resorts that come to mind are Breathless resorts. Breathless is a chain of adults only all inclusive resorts that are part of the AmResorts brand. These resorts were created to fill this gap for fun, party adults only resorts. Breathless has resorts in Mexico, Jamaica, and the Dominican Republic.
Breathless Riviera Cancun is my favorite, but it's not for beach lovers. The dining is great. I love that there are two pools - a pool with activities playing top 40 hits and a second pool with more Club music and pool parties. If you want your room to be a bit of an escape, you can book your room at Secrets Riviera Cancun. The Secrets is connected to Breathless.
Breathless Cabo is right in the marina. It has two separate buildings. One building has the rooms and an activity pool. Across the street is the Beach Club building with most of the restaurants, the party pool, and the Spa. The marina views from the rooms is my favorite.
Breathless Montego Bay is located next door to Secrets Wild Orchid and Secrets St James. It has a small beach club and a large party pool with exchange privileges to Secrets. Secrets Wild Orchid is another good choice for an active, fun adults only resort.
Breathless Punta Cana has the best beach. The party pool is the smallest here, but the activity pool is long and winding. The restaurants are very good and set up for group dining.
Breathless Cancun Soul is scheduled open in December.
Other Adults Only Resorts with nightlife with more mixed reviews:
Grand Fiesta Americana Puerto Vallarta - The rooftop bar here is very busy in the evening.
El Dorado Royale - The evening show segues into a DJ / nightclub vibe.
Hard Rock Riviera Maya Heaven Section - During part of the year, the Heaven section is Adults Only. It has weekly fun pool parties and DJs. On the weekends, the night club downstairs opens up and it is really fun. The other nights, there are live bands and DJs to keep the party going.
Sandals Ochi - This is the first resort where I saw a night time foam party. The resort has a pool that's not close to the rooms so they can have some really fun night time parties. You have a choice of several pools to enjoy during the day so you can find the one with the right level of activity.
RIU Republica in Punta Cana isn't quite as upscale as the other resorts, but it has a water park! The pools and nightlife are definitely active.
One warning: Nightlife requires people. These resorts do a great job creating an environment for a fun pool party or night at the club, but if everyone there is going to bed after dinner, you may not have that experience.