I say elephants. Not only are they able to completely thrash any other land mammal in their normal state, but in a blind rage, they would totally destroy a polar bear.
Elephants have been seen sending hippos tumbling…
…giving lions a run for their money…
..and overturning massive vehicles.
While on the other hand, polar bears seem minuscule next to these beasts:
The polar bear will have a real hard time biting through the elephants extra-thick skin even if the elephants goes AFK for a while. The elephant will also be too heavy to push or move in some way. I think to kill a full grown elephant, about 5 polar bears might be able to do the job. Against a raging elephant, possibly 10 might do it. That is, if the fight is set in a cold habitat, suitable for the polar bears.
I don’t mean any disrespect to polar bears, who are one of the most dangerous mammals on Earth, and will fare very well faced against most other apex predators, but elephants are simply out of their league.