Not sure about the most beautiful in the country, but there's a moorland valley called Bransdale near where I live. Nice picturesque countryside, with some woodland and farmland in the valley, an old mill by the river, with moorland on 3 sides as you go up and out of the valley.
It's also very quiet and undisturbed as there are no main roads that go through it to anywhere, just mostly single track country lanes that go into Bransdale and back round in a loop. That plus things like lack of facilities like car parking (so you have to walk for a while if you want to explore) means it's not a big touristy place. Definitely one of my favourite places in the national park (North York Moors).
Can't upload any of my own pictures right now because of internet problems, but googling it should bring stuff up.