From all the beaches on Kos, I enjoyed Mastichari the most. We stayed 10 days on the island, so we had plenty of time to see many beaches and villages, but Mastichari seemed the best to me. People were younger, beach bars played loud music (without the craze and noise of Kardamena!!) and you can windsurf there because winds are very strong in the afternoon. Windsurfing is my favourite activity in summer and most Aegean islands are good for that.