I shot at least one elk a year since 1958 and have used .243, .30/06, 7x57, as well as a 7mm Remington Magnum and a .308 Win. The sole issue was that each of these rifles were solid .5moa shooters and I took no chances by shooting a ranges that I was uncertain of. As has been stated above, bullet placement counts. As to my favourite bullet, for the bullets:
.243 it was a 100 gr Nosler partition
.30/06 it was a 180 grain Speer Hotcor
7x57 it was a 160 grain Nosler Partition
7mm Rem Mag it was always a 160 grain Speer Hotcor
My recent elk killed was a .308Win using a 180 grain Nosler Partition
(this rifle loves the Hornady 178 grain A-Max bullets too, and I would not hesitate taking an elk with one if at a distance somewhere greater than about 350 yards out.)
Never lost an animal in all that time, and with the exception of the last couple of years, I also took a moose and a white-tailed deer with those rifles each year as well, not to mention other species using the .30/06 like caribou, grizzly, black bear and in the early days when I still had it, my .243 was deadly on mule deer as well.
I would have to say if I were to pick just one of those cartridges it would be the 7mm Remington Magnum in a steel epoxy and pillar bedded Husqvarna Crown grade topped with a Leupold scope, which is presently a VX-3 3.5-10x44. This rifle has shot .375 inch 5 shot groups at 100 yards using Hornady A-Max bullets when I am up to it.
