I have two Favorites. Cedar Point and Six Flags Great America. Both for different reasons. Cedar Point is a ride park, and has a lot of really good roller coasters and a few older rides that are available at few parks. My favorite ride there is the Raptor, which is an inverted roller coaster.
As for Great America, it has more for your buck. It offers some good entertainment, and has a great mix of rides and thrill rides. Even though Cedar Point claims to be the roller coaster capital of the world, Some of Great America's roller coasters are far better than most of the "count" at Cedar Point. And, most recently they added a huge water park to the property, which is included in your park admission, which is a heck of deal.
Unfortunately, in their rush to make mega bucks. the park owners are not giving us the thrills that came to us in the 90's. It's more what upcharge attractions can we bring in, and how can we make more money off from our guests. Like allowing guests to pay to cut in line, which before that, line jumping could get you banned from the park. Now they encourage it. Sad times.