Well all theme parks are really busy in general, but most times I went it was practically packed. Definitely recommend going, it's money well spent, especially if the weathers good
Definitely Thorpe Park in the UK, but theme parks in America are waaaaay better.
I am from the UK, I travelled to Sandusky, Ohio in America to visit a theme park called Cedar Point. It is widely regarded as 'the best theme park in the world'. It's home to that record breaking 'Top Thrill Dragster'. Highly over rated. I was sat at the front, and it feels exactly the same as Stealth, in Thorpe Park, even though it's more than twice the height (exactly the same ride design).
I love Thorpe Park, Alton Towers, Chessington, Drayton Manor and Oakwood, and I'd really like to get to Lightwater Valley and Blackpool at some point too