Thirty aught six.
I don't own a hunting rifle, but from the time spent with my dad's Remington 700 BDL, I love the .30-06. That roar and thump let you know you sent something serious downrange, but it's manageable enough for a fast second shot if the first failed.
Extremely widespread and versatile too. Kinda like the rifle version of the .357 magnum in handguns.