Killbear, Sandbanks and Mara are my favourites. They all have long sandy beaches with gradual depth. You have to book early 5 months in advance, for Killbear and Sandbanks though.
I have been to the beaches in Sandbanks and Wasaga. Both of them have nice shallow beaches that area ideal for swimming. Sandbanks Provincial Park have campgrounds that you can reserve but Wasaga Provincial Park doesn’t. Most people who go to Wasaga opt for cottages or private campgrounds nearby. There is also a lot of picnic crowd in Wasaga.