Honestly - all of them!!

Whatever makes you as a parent happy will make your 18 month old happy. They all have fun things for little ones. They all have nice pools.
We were at VWL in Jan. with our 17 month old and he had a great time exploring our villa and the lodge. We enjoyed the boat ride over to MK and the closeness to CR for CM's and also FW to do the petting zoo and pony ride for DD who is 5.
We have also stayed at BWV when DD was a toddler and loved it also. Of course the close proximity to Epcot and MGM are awesome. DD loved the entertainment on the boarwalk in the evenings and we loved the options of restaurants and nice walks around the boardwalk.
Also while DD was a toddler we did a stay at OKW that was also great. We were right next to a quiet pool and playground area that was perfect for DD. Whenever we stay at OKW we make sure we have a car and its so nice to just roll out of the room and get on our way in the car.
In July when DS will be 23 months old we will be staying at BCV and I know SAB will be a big hit with both DD & DS.
Go where you'd like to stay!