...if you want the beautiful wide, long white sand beaches, go to Punta Cana/Bavaro...
My overall favorites are Playa Grande, Bahia Las Aguilas, Playa Rincon (more for the view as you head there than the beach itself), and of course, Cabarete. I think the most "fun" beach is the main beach in Sosua...its a protected bay so the water's usually calm, great for swimming or snorkeling, there are almost always lots of people, and tons of little restaurants, bars, gift shops...some find it "tacky", a little run down, or too touristy, but it's an experience and full of life...