The best waterproof swim cap is the one you wear regularly when in the pool. Silicone, neoprene, latex, and Lycra caps all claim to keep your hair dry. Silicone gets the most positive feedback from the consumer.
Silicone swim cap wearers swear that their hair is kept dry when they wear this cap. Latex cap wearers state that their hair is mostly dry, which suggests some water can get under the cap. Lycra breaks down when exposed to chlorine, so it has a shorter lifespan, but it does keep the hair dry when worn. Neoprene cap wearers wear them to keep their head warm. It is not known as a cap that keeps the hair dry. These neoprene cap wearers also wear a latex, Lycra, or silicone cap to underneath the neoprene for that purpose.
Depending on your goals, the right swim cap is available in either silicone, latex, Lycra, or neoprene.