Cold!! I thought about musk ox until I read about the ride in the snowmobile trailer.I guess I'm kind of in the minority here with @TOM. While they are a cool animal (aren't they all?), they are likely the least appealing to me of all the mountain species. I think the sheep species, our NA mountain goat and the tahr are all more attractive, but the most impressive for me is the different ibex varieties. They are incredible looking creatures. However aside from having a lunch bucket budget there is one other thing that would prevent me from hunting the sheep/goat species and that is sheer terror of falling to my death up in those mountains! Makes my blood go cold!
If I were to pick an extreme hunt that would be more for me, I think it would have to be muskox up on the tundra. That would be cool.