We found Loutrou and went there by ferry for a fantastically wonderful day. I can't imagine any better beach in Crete.
This would be on my list of places to return to and explore the coast further about 6 years ago
For superb try
SW Elafonisos is unique, NW Balos (have to get a small boat), if you have a day to spare must visit Chrysi island from Ierapetra SE have been told that the beach is exeptional.
Also if you pack a tent do visit Gavdos island from Plakias.
Most of the rest beaches all around crete are very good depending on the amount of people on the beach.
Enjoy about 6 years ago