I think the best beaches anywhere are warm, sandy and not too crowded, wherever you can find them... Vancouver, Vancouver Island and along the Pacific coast in general...
I also experienced an almost isolated sandy beach in Ecuador, as a child with my family for a week, which was amazing... There was a local village nearby...
Some wonderful beaches in the Carribbean...
I dream of visiting Hawaii, or Spain, or UK (they must have some great beaches!), or pretty much anywhere in the Mediteranean...