The most popular attraction is the Autopia ride (you can find this and other statistics about Disnelyand here). This is a car ride where you steer your own car along a special track. The name comes from 'automobile utopia' and it is one of the oldest Disney rides, having been established in 1955. It's still quite a simple ride compared to some of the others, but children love it and will form long queues to go on it.
Haunted Mountain, Space Mountain and Splash Mountain are the next most visited attractions. In fifth place is one of the most internationally famous rides,and in many people's opinion the best: the Pirates of the Carribbean ride. It consists of a boat ride through eerie and 'dangerous' landscapes featuring adventures and pirate attacks. It has been much copied and was the inspiration for the film series of the same name.
All of these rides are so popular that visitors are advised to get a special Fast Pass to avoid long waits. On the other hand, the least popular attraction is Tarzan's Treehouse, so you don't need a pass for that.