I prolly shouldn't even say, as I haven't been to so much of this country, but...so far, I would say the north coast of Cornwall--around Boscastle down towards Tintagel and all the coast along thereabouts: it's the most beautiful area I've ever seen, just breathtaking.
And, this may seem silly as it's a singularly small place, it doesn't qualify as an area, per se...but I love Heale Gardens, which are a few minutes drive from where we live. I just find it so lovely and peaceful...
And Stourhead, too...