I'm surprised posters are putting whole counties and, in some cases, whole countries. These places won't be uniform (unless if really is that awful). North and South Devon are quite distinct from each other and Devon's tourist spots like Dartmoor, Torbay, Dartington, Exeter, Saunton Sands, Clovelly, Plymouth, Buckfastleigh, etc, are all very different places and experiences. I just happen to mention Devon as I'm from there.
We love Cornwall, especially the Lizard Peninsula, but Newquay is a dump with some lovely beaches nearby. Both Devon and Cornwall are pretty rural, so if you get wet weather, you'll be pretty limited.
I think rubbish weather will effect your perspective on how you view a place.
I was disappointed by:
The Lake District
North Wales (can't remember exactly where, have erased it from my memory), but it rained the whole week we were there, so didn't see it in it's best light!
Sherwood Forest
Lundy
Portland Bill
We drive through Sandbanks occasionally and, yes, it is pretty underwhelming.
Can't believe anyone would put the Isles of Scilly!