Who would win a fight between a hippo and a grizzly bear?

Hippo. Hippos are actually the most deadly animal in Africa, and are so much so that even a pride of lions would think twice about tackling a hippo cow or bull. A hippo's jaws are extremely powerful, and will kill a bear instantly, like it has with humans that have had the unfortunate problem of coming too close to an angry hippo.

Another answer:

Although a hippo has thick skin to protect itself from a grizzly bear's razor claws and deadly teeth, the hippo also has a worse aggressive behavior of killing humans and flipping down boats. Hippos have been known to kill large nile river crocodiles for protecting their young and themselves.