The best co-branding is when one brand is not too dominant. Both should gain equally by joining forces. Similarly, both should lose out if the co-brand is removed.
On these criteria the example on the right ticks all the right boxes for me. Remove the Müller Corner and the Kellogg’s Coco Pops have no leg to stand on. Take out the Kellogg’s Coco Pops and the kiddies have little left to look forward to.