From the ones i've been to, OKW pool. Though i'd love to go to the Y&BC pool, but it's for resort guests only (you know that DVC members can "pool hop", you can't at this pool or AKL pool...)
But my favourite isn't in WDW, I really enjoyed the Vero Beach Resort pool. It was large, they hold scuba diving lessons there, the slide was extremely fun and fast, etc.