I believe the American top five are:

Kodiak Brown Bear (Or Grizzly)
Rocky Mountain Elk
Mule Deer
Sheep (Any of the four species)
Moose (Shiras, Alaskan or Canadian)

I have been fortunate and have harvested all five. If someone was to offer Pronghorn, Whitetail, Mountain Lion, or Mountain Goat to the list, I would be hard pressed to argue against.