What about General Mills, the only brand to have all their cereals made with whole grain, and the maker of Cheerios, Lucky Charms, and Cinnamon Toast Crunch? :| And they own Betty Crocker, Green Giant, and Pillsbury? Plus they have the Box Tops for Education deal.
Or Malt-O-Meal, which has cheaper versions of popular cereals, and the only one to make blueberry muffin cereal that I know of?
Still, I have to go with Kellogg's, only because they are the ones to make my favorite cereal ever, Cracklin' Oat Bran. Also, they have Eggo waffles as a subsidiary, and those are delicious. <3
But Post makes Honey Bunches of Oats, which is also delicious. But they don't have any other food companies. So for the sake of the poll, which I can't vote on because it's not multi, I'm going to go with Kellogg's.