I think that would have to be Colwell Bay on the Isle of Wight. It's a five minute walk from my parents' house and it's classed as one of the best beaches on the Island. In the Summer, it gets busy enough to have lots of atmosphere, but having only a handful of small cafes and bars, it's not tacky and naff as a lot of the larger, more touristy seaside resorts around the UK are.
A couple of pictures taken at quieter times:
Looking to the east
And then to the west, all the way round the bay out to the Victorian fort, Fort Albert on the far headland.