Most people tend to pick a larger than necessary caliber. Most novices think that all elk shots will be farther than 300 yards, that isn't true. What's more, most novice elk hunters over estimate their skill and fantasize about taking a giant elk at 1000 yards from hillside in the Rocky Mountains.
The truth is that it depends on where you will be hunting, your kill could happen anywhere, brush choked draws to wide open plains. I hunt in heavily treed mountainous areas and most of my shots are under 350 yards. In my case I feel comfortable with my .308 Win rifles, I use a Kimber Mountain Ascent or one of my semi-auto rifles. I have no fear of taking shots out to 350 yards with my rifles and with the Kimber I'd go out 600 yards with the right shot setup.
You'll hear a lot different answers but if I wanted one rifle that would take an elk under any circumstance then I'd probably look at a 30-06 or a .300 short mag. You can push both to pretty high velocities with heavy bullets or you can down load them to shoot some light loads for close range.