Alton Towers, I would be prepared to bet, probably has the worst opening times of any major theme park in the world.
In my 2011 Holland/Belgium/Germany park tour, all parks except one opened later than Alton Towers - some by up to 6 hours longer in the evening.
In 2012, on my European Park tour, all parks opened later than Alton Towers - some by up to 6 hours longer in the evening.
In 2013, the trip I have just done - here are the opening times:
Coney Island (tiny, seaside park): 11:00am - 11:00pm
Six Flags Great Adventure: 10:30am - 10:00pm
Lake Compounce (small family park): 10:00am - 8:00pm
Six Flags New England: 10:00am - 10:00pm
Busch Gardens Williamsburg: 10:00am - 10:00pm
Kings Dominion: 10:00am - 8:00pm
Cedar Point: 10:00am - 10:00pm
Kennywood: 10:00am - 11:00pm
Hersheypark: 10:00am - 11:30pm
Knoebels: 10:00am - 9:00pm
Dorney Park: 10:00am - 10:00pm
Yeah, Towers sucks.