I've only been to Chessie, Thorpe, and Alton, but I have to say Alton blows all three of them away.
Thorpe is actually a decent park though, a lot better than anybody gives it credit for, although people say I caught it on a good day. The coasters are all pretty average, but it's not a terrible park by any means. I've been to a lot worse, and have been on a lot worse rides.
Chessie is definitely more of a family park. They lack as far as coasters go, but that's okay because their theming is pretty nice and they have animals, etc, and you can still have a laugh there without major thrills.
Alton just has everything. It has the scenery, the theming, the good coasters, enough to fill up an entire day and then some. I loved it, and it's in my top three parks, without a doubt, and perhaps even my favourite park I've ever been to.
So, Alton.