First, what is 'best' for you might not be 'best' for somebody else. So, think about what your priorities are, and what 'best' means.
We like Mexico.
And, other than Beaches, Mexico has the biggest choice of All Inclusive resorts.
As mentioned, it can get expensive if you want the space and extra beds.
Beaches on Grace Bay at Turks and Caicos would be the absolute dream for a kids family vacation.
But, for a huge, huge, price. $$$$.
We are returning to the Royal Sands, which is a Royal Resorts, condo resort. (originally it was totally time-share)
They have one bedroom units with two Murphy Beds in the living area, full kitchen, etc. And, two bedrooms as well.
BUT, they are not resort with the fun factor. VERY nice resort! Good food. But, not like pool parties, etc... like you might be imagining.
If you have not, go over to tripadvisor. Look up Cancun and the Riviera Maya.
All the info you need is there!
You can find good info on wlhich resorts might offer larger units.
I might look into the Iberostar resorts, at Cancun and on the Riviera Maya.
There is the Grand Palladium, which is south of Cancun on the Riviera Maya.
The Barcelo resort complex, which is down close to the Grand Palladium.
Dreams resorts are family friendly.
At Cancun, you can look at other resorts like the Hyatt Ziva, Hard Rock.
Riu resorts can cost less.
But, I am not sure that this would be the way I would go.
Watch out for TWO things:
ONE: Is the beach a nice beach, like what you are probably imagining.
Not all beaches are equal.
In Mexico, beaches along the North-South stretch of the Hotel Zone will be that gorgeous windex blue beach. (But with high surf)
Baaches on the East-West section, or Top of the '7' shape, are calmer, but not like the beaches just mentioned.
Beaches south of Cancun are hit and miss. Some are just not as nice, rocky with 'ironshore' (rock and coral)
TWO: Do they a limited number of suppers at the full service ala carte restaurants. Which usually means making reservations which can be a hassle.
(Palladium has unlimited dining, with no reservations. Many, if most, resorts are limited.)
Decide on about three specific priorities, then go from there.
For example, your priorities might go like this.
1. Large Unit to sleep everyone comfortably.
2. Activities and 'vibe' at the resort
3. Great beach
And, a 4th might be food and dining, or near airport, etc.
If great off-resort activities might be important, Mexico is the place to go, Hands down. Snorkeling, Tulum Ruins, Adventure activites like Zip-lines and Cenotes, etc.. etc...
By deciding on the biggest top priorities, it will be easy to narrow down your choices on which resort.