I would have to say mountain goats.

On any given day any species may feel impossible or extremely difficult, especially when you throw in trophy hunting, weather, and weapon choice. I used to think elk were the toughest but to hunt goats, regardless of weapon choice, you climb right on passed the deer, moose, elk and sheep country. You keep on climbing and eventually you're in goat country and only then can you even start looking for them much less be hunting them.

Imagine climbing up to a 12 to 13 thousand foot peak just to realize the goats are on another peak meaning you have to descend maybe 5 to 7 thousand feet and reascend. Then you try to get into position and the goat is either gone or spooks and you have to do it all over again. That's just opening morning.