Depends how long you'd be going, and whether you want a theme park or a ride park.
As a vacation destination, and the best theme park, Walt Disneyworld. This place is awesome. You could easily spend two weeks in all the parks there. It's definitely the best.
Day trip theme park, Universal Hollywood. Not as big as the one in Florida, but I much prefer it. It's actually a movie studio in Hollywood. Like, at one part, there's just a chain link fence and then soundstages and studio stuff going on.
For roller coasters, Six Flags Great Adventure. All Six Flags are good, but Great Adventure has Kingda Ka, the biggest roller coaster in the world.