Right now, 338/06, but I really like my 6.5x06, but I really also like all 3 of my 30/06's. I have also had 25/06, 270,280,8mm Gibbs, 35 Whelen AI, 35 Brown Whelen and liked them all at one time. And I can't forget my 35 Whelen that I still have.
I have killed or seen killed mule deer, blacktail deer, caribou, bighorn sheep and elk with a plain old 30/06. Somewhere I saw a picture of what was supposed to be the world record brown bear that showed it was killed with 30/06. If I had to own only one rifle it would be a 30/06.