Our favorite beach hands down is Crash Boat, in Aguadilla. That is, if you're not counting Flamenco beach in Culebra, as mentioned above.
Crash boat has a calmer area, good waves on one side, a pier to jump from, snorkeling, great paddle boarding and kayaking conditions. It's an all around great place to spend the entire day. There are also food kiosks, plenty of parking and showers.
We also like Shacks beach in Isabela for snorkeling.