It looks most of this thread was active before many of us joined Hunt talk, but the question still holds...
I shot my first elk in 1965 with a borrowed .30-40 Krag. Back then I was working summers in Steamboat Springs, CO, and the next summer I wanted my own elk rifle so I asked a couple of the locals that I worked with, what caliber rifle to get. They said either a .30-06 or .270 and that only the city dudes in Denver that came to the mountains once a year used .300 Winchesters.
So I bought a .30-06 and that rifle easily put 8 elk in my freezer. In the late '70s I had that rifle re-chambered to .30 Gibbs and then that rifle put another 21 elk in my freezer.
I've shot elk with other calibers and I've had friends that shot their elk every year with rifles chambered in .22-250, .243, and up. It mainly depends on where the bullet hits the elk.
My favorite elk rifle now is my .300 Weatherby shooting 180 grain Barnes TTSX bullets.