= Definitely Alton Towers.
While Stealth, Colossus and Saw are good coasters, if you have been on Nemesis Inferno after having been on Nemesis in Alton Towers, you will be disappointed - I certainly was!
Alton Towers is more spread out, and it really feels as though you are somewhere big; there is more choice of rollercoaster. In fact there is more choice of all sorts of rides, so if you have a friend who is not a thrill seeker, there is more than enough to keep them occupied.
Thorpe Park is more compact, and actually more confusing in my opinion; the map isn't a great help, and you can end up at a dead end, with your intended destination on the other side of a 6 foot fence!
Alton Towers is connected by a sky ride which can get you from one side of the park to the other in a matter of minutes, and is actually quite relaxing if you're not scared of heights - you get a great view from up there of the gardens and other rides.
I had a good day, and I'm glad I went to Thorpe Park, if only to reinforce my opinion that Alton Towers is head and shoulders above it (and obviously Chessington!).
Hope this helps!