Holiday World is my personal favorite, but everyone should go to Walt Disney World at some time in their life or another... just don't expect the kind of coasters you see at the big parks. Universal is a great mix. You get really unique rides, and the theming is just incredible!
Cedar Point's also another one that will blow the mind if you've never been there before. I've only grown cold to it because of how much I've worked there. If you're looking for coasters, Cedar Point. If you're looking for unique experiences, Universal or Disney.