I've only been to 3 beaches so far... (I know, very few!).
The one I liked best was at Garda Lake, the Lungolago Mazzini beach, of Peschiera del Garda, to be precise. It's not a big beach, but it's pretty long. And you can choose whether you want to sit on the sand beach, pebble beach, or on the grass.
The prices are fair as well, I actually have my own stuff (beach chair + beach umbrella), but you can rent them at very low prices! For €12 you get 2 beach chairs and a big umbrella
I think they have pretty much the same prices around the lake...
I also loved this beach because you can either sit among the crowds, or you can choose a secluded space, like on the grass under the trees. They also have a tiny kid's park, if you plan on bringing kids. Oh, and the parking is right there, you don't have to walk a lot to and from your car.