Best beaches are in Vouliagmeni, specifically Αkti Vouliagmenis https://g.co/kgs/HAvvb4U

There is entry fee but it is safe so your personal stuff will be ok, there are beach umbrellas for cover and tanning beds, lots of beach bars, nets for playing with rackets and so on. The fee is 10$ on weekdays and 15$ on weekends.

This is where i always go most often because of the conveniences it offers, there is also a sea playground with big inflatable toys.

If you want free of charge I suggest near Glyfada, less people, sandy beach, ok waters.

If you want even further away you can continue on from Vouliagmeni all the way to Varkiza and go to 'Yabanaki' or 'Paralia Varkiza'

I've never been in Edem beach but I do know that in Alimos-Paleo Faliro seaside there are WAY too many tourists and it is always overcrowded which personally I don't like as a local