>>4997173 (OP)Considering the Apex of both species, the Hippo would win.
The weight alone would be a difficult wall for the Bear, the avarage is around 450kg for the bear, with the heaviest registered being 1 Ton, meanwhile the Hippo on avarage weight 1.5 Ton, with the heaviest registered that weighted above 2 Ton and the heaviest known (but unconfirmed) was one that weighted above 3 Ton.
Then it comes the skin, 6cm thick for the hippo, the avarage lenght for bear claw is 7,5cm with some that measured even 10cm, it would make some damage since they don't have a fat layer, but nothing that would be really fatal (they fight against crocodile which have nearly 13cm long teeth), the bear teeth do not matter, they are 4-5cm long, they would scar the skin but not penetrate it. On the opposite side you have the Hippo canine and incisors which can be nearly 50cm long, even tho the Bear have quite the "armor" (5cm very dense coat, 10cm thick skin, up to 12cm thick fat layer), they would make some nasty damage, creating hole in it without much effort.
For last there is the Hippo mounth, they have a very large mounth which can embrace the whole polar bear head, the strenght (up to 2000 PSI) would be able to crush most of their bones or make some major crack in their skull.