I've been thinking on this since you first posted the thread. I have too many favorites depending on the specific application.
In the end though, even though I don't currently own one I came up with the 7mm Remington Magnum.
It's simply the most versatile caliber out there for everything from prairie dogs to Elk and Moose, and very cost effective to shoot.
Tolerable recoil in a light rifle, capable of .25 MOA accuracy, flat shooting/high velocity, with a great selection of bullets for everything from small varmints to all but the biggest and most dangerous game.