Agreed. Magic Kingdom is the most iconic, and in our opinion has the best breadth of attractions and variety. To our family, it epitomizes the entire Disney experience. We always try to start our vacation there and then spend our last night there. Just something about that castle...
Of course, it also depends on your family's tastes, whether it's your first trip or your fiftieth, what the park hours are that day, etc., but for us it'd be MK all the way.