The .30-06 is hard to beat, it was my first high powered rifle, and it's been my stand by (Ruger Mark II Model 77).

Honestly, looking back knowing what I do now, and planning for ONE rifle ever (thank God I don't have to live with just one!), I'd pick up either a heavy barreled .308 in stainless and laminate, or a standard barrel .270 in a standard sporter.

I'm a heavy barrel nut, and the .308 will take down absolutely anything on the north american continent with ease, AND is easier on the shoulder than say the .30-06, or .300win mag. With a heavy barrel and the punch of a .308, I can reach out over 400yrds with confidence on anything from coyotes to heavy whitetails (probably want to be a little closer on elk).

Passing on the heavy barrel (PRICEY), I'd pick up a .270. A little lighter recoil than the .30-06, .300WM, 7mm Mag, etc, but plenty of punch to take out even heavy Elk, or even moose, but still light enough to limit hide and meat damage on lighter game like coyotes or antelope. The .270 is a shooter's cartridge. I think it's one of the most underrated cartridges running.

If you have the money, then heavy barrelled rifles are worth the extra expense, and since you're limiting yourself to only ONE rifle, then I'd strongly recommend a heavy barrel. I personally favor stainless steel and laminate stocks. Mount a bi-pod, a very comfortable sling (double shouldered preferably), and a 4-12x50mm glass, and you'll be able to take down ANY north american game animal that comes within eye-shot. This set up in a Savage rifle, with a harris bi-pod and a budget conscious Bushnell Elite scope will run you right around $1100-1200.

On the more budget wary side, a standard rifle in blue/walnut chambered in .270 with a budget (yet quality) 3-9x40mm glass will run you about $800-1000. Personally, for the money, I'd rather have the heavy barrel, but the standard rifle is easier to carry and a little easier on the wallet.