The best beach is Medano Beach, in my opinion. It's safe to swim there and there are all kinds of water activities, restaurants, bars, and vendors.
Chileno and Santa Maria are beaches on the corridor that are nice, and have smaller crowds if one wants a more secluded type of beach.
The beach on the Pacific side is awesome to look at--from a distance. There are dangerous waves that make it unsafe for swimming and often unsafe for even getting very close to the water. People have drowned from being hit by rogue waves and dragged out to sea.