Hi Supervaca,
It really depends on what you are looking for in the way of a beach. For surfing the best beaches are Sunzal, El Zonte, Punta Roca and El Tunco. For swiming on the El Salvador coastline one must be very careful and a good swimmer as we have some very strong north bound undertows. My favorite beaches for enjoying the water and sun are El Tamarindo, you can find information at Tamirindos signature above, La Puntia at the end of the Costa del Sol and Playa El Espino a beautiful black sand beach that on weekdays is very desolated. For more information about hotels and restaurants on the beaches check out the site in my signature. If you need more details feel free to write.
Saludos form Suchitoto
Robert Broz