All of them easily reachable from the city and well visited. The last 4 although further away are the most visited by Britons. However, Ammoudara is for more family oriented people.
BTW, the North winds which are prevailant in the area will most certainly get windy and choppy seas, but Ammoudara is somewhat protected to that since it faces east.
For me the best beach is after the archaeological site in Mallia, it's sandy and more quiet. But i should warn you, if there is north wind all the beaches have waves, not my favourite style of swimming!!! In that case, you can to go to the south part of Crete...
Go to Stalis - drive along the through road until you come to a hotel set back called The Moonlight - opposite that you can walk through another hotel to the beach... it is fantastic. Great place to sit and eat also next to it.
Ligaria, aprox 15 km from Heraklion is a lovely cove with a beautiful sandy beach.
You can find the best Cretan beaches via the links below.
https://www.google.co.uk/#q=best+beaches...
better bring your best dancing boots - gonna need them
https://www.youtube.com/watch?v=GWbUXGSE...
Ηρ?κλειο Krete
https://www.youtube.com/results?search_q...
Weather in Heraklion
https://www.google.co.uk/webhp?tab=ww&ei=eFrrU-LqNYnWPcvAgcAN&ved=0CBUQ1S4#q=weather+in+heraklion
Look what happens when you eat too many olives!
https://www.youtube.com/watch?v=HMOLNjdQF90&hd=1
