Well if we're just suggesting UK parks, I would say Blackpool Pleasure Beach is the best in my opinion... JUST!
The park is making big improvements despite struggling at off peak times. Wallace and Gromit was a great new addition and there have been plenty of general clean ups around the park and re-development of rides like Steeplechase.
The reason I say "JUST" is because Alton Towers has been great this year and I think it's a shame that some other forums (naming no names but hopefully people know which one I mean) feel they have to resort to making 'digs' at Alton and Merlin to make their park look better. Smiler was a good new addition and Alton Towers' coaster line up is superb, despite being too gimmicky for my liking. In previous years, I would have said Blackpool straight away, without question. This year I've been impressed by Alton Towers....... Sadly I've been pretty disappointed with Drayton Manor this year, although I've not visited since May so things could have improved. Chessington has also been quite a let down, as I'm normally quite a fan of that park.