If I was to limit it strictly to 250lb mule deer or smaller, I'd probably have to go with the 7mm-08. It is a flat shooting, light recoiling, accurate round capable of taking even elk out to about 300 yards or so.
If I was to include the possibility of taking elk or moose with that one gun, I would probably have to go with the 300WM or 300WSM. However, since I don't own either of those, my vote for "best all around deer gun" goes to the '06 because that is what I have and it has never failed me, even for elk out to 350 yards.