You can expect a lot of answers on this one. Ideal conditions are one thing, less than ideal conditions and the answers will move into the relm of elephant guns.
The best answer I think is a firearm that you are comfortable shooting in the 7mm mag. 7mm ultra, or higher end .308's (30/06, .300WM, .300 ultra, .338, .375 and those that fit in between these.
People do use 7mm mausers, 45/70's, .416's, .458's and I have even killed one with a .44 mag.
A good quality bullet should go hand-in-hand with the caliber.
The guns in my safe that I would use are my 7600 in .35 Whelen. NEF Ultra in .35 Whelen and my Contender in .445 Supermag. My .308 is super accurate but I feel a little light in less tha ideal conditions.
Lets see what the others suggest! :-)