Two of my favourite beaches in Malta are Golden Bay and Ghajn Tuffieha Bay, they both sit next to each other and are only walking distance apart. They are situated on the Western part of Malta, with Golden bay having most of the facilities, like places to eat and toilets and the large 5 star Radisson Golden Sands Hotel and thus this beach gets much more populated especially in mid summer. You can hire all sorts of equipment for watersports from this beach and there is a diving school also located on site.
If you are looking for an un-crowded beach, somewhere to get a little peace and quiet Ghajn Tuffieha Bay may be just what you are looking for, this beach is also great for snorkeling. Because Ghajn Tuffieha Bay does not have any facilities, it tends to be much quieter and though it cannot be described as a nude beach, I have seen nudists at the far end of the beach before. If that is not your thing, you can easily stay away from that area and you would probably not even notice them.
You have to climb over 100 steps or you can choose to climb down hillside paths to get to Ghajn Tuffieha bay and so would not be suitable for anyone who has dificulty in climbing. To get to Golden Bay and Ghajn Tuffieha Bay you can take the bus Route 47 from Valletta or alternatively route 652 from Sliema.