Viareggio without a doubt. But don't forget that most Italian beaches are 'organised' ie private, so you pay a fee to get on them and use a sunlounger, changing cabin and a shower. Sometimes there's a bar or a little restaurant. Obviously there is public access to the beach, but it's usually just a walk way with gives you access to the water's edge and about 2 metres of sand behind it. The region you are interested in is often referred to as the VERSILIAN RIVIERA - try googling that and see if you get any other info.