Done two of them, and quite enjoyed both of them, probably because I've only had one visit each.
1. Alton Towers - Love the setting. There's some really nice areas including places that don't have any rides, like the open field or the gardens. The whole park honestly felt special to me. Not the best ride selection, but I like Air and Oblivion, and Nemesis is amazing. Could honestly ride Nemesis all day.
2. Thorpe Park - Feels like a Six Flags park, but I actually left the park feeling pretty good, whereas I've had times where I leave a Six Flags park unsatisfied. I had 4 hours at the park and had a pretty decent time, with rerides on the good stuff and queues that weren't bad. Stealth, Inferno, and Swarm are all great. Colossus and Saw are fine but I'd ride them again if I ever return.
Chess does boast the best spinning coaster in the UK and the best suspended coaster too. That's two reasons. Alton Towers is very well coaster orientated and miles better than Chessi with choice of extreme rides and I'm happy to concede that but 'WOA' does have an unique ambience all of its own.
For just coasters Alton towers easily, But as an all round park Yea Chessington is my Favourite. When i go to Alton its all about coasters and Hex but at Chessington i find there is much more verity. While there has been updates to classic rides which are not as good as the originals they keep them going, DF and Vampire are great and don't get enough love and the zoo is always interesting to walk around.
Nemesis is still my favourite invert but all the outer coasters at Alton are either megh or i have got to go on a better versions of them so they do not hold up well, Chessington however still has my favourite suspended and spinning coaster. I also went here much more growing up so i have much more nostalgia from this park.