Been to so many good places that's really hard to narrow down. We've completely enjoyed every location we've been to, and every campground has it's own uniqueness.
Last Summer we were in South Dakota and stayed in Custer, SD. The entire area, the Black Hills, and the Badlands are absolutely great! Most of the private campgrounds are all about the same. We stayed at Buffalo Ridge RV resort (which is just a Good Sam campground) in Custer, SD. But the really neat and awesome campsites were inside Custer State Park. So, Custer State Park is tops in my list.
We have visited EVERY state park and state recreation park in Indiana and every one of them is fantastic! Each is unique in it's own way.
We've also stayed at several State Parks in South Carolina, Georgia, and Florida, Kentucky, Tennessee, Ohio, Illinois, Minnessota, and Wisconsin over the years and every one of them was great! Every one we went too seemed to be the absolute best from all the ones before. Looking back, all of them were great!
State Parks anywhere in the mid section of the Country and you can't go wrong! The only one I would NOT recommend to anyone is Boonsboro State park in Kentucky, as it located adjacent a very noisy highway beside a stone quarry. Big trucks use their jack brakes coming down the hill and the stone quarry fires up very early in the morning. It's constant noise and there's no escaping it! That is the ONLY state park we've ever been to that was disappointing (because of the noise). Had it not been for the outside noise, it would have been a phenomenal park also.