I'm the first shooter to agree that no-one should rely on a "all-purpose" rifle. I don't think there is such a rifle. To truly be an "all purpose" rifle the weapon would have to bring down everything from cape buffalo to ground squirrels and do it effortlessly without undue work and industry on the part of the shooter. I would submit that the best anyone can do is find several calibers they like that are "multi-purpose" rather than "ALL-purpose". Many claim the .243 and the .270 and the 30/06 and the .300 win mag (and others) to be magic calibers that are "all-purpose". If I had to pick one caliber that could really fit the widest range of "multi-pupose" [in the middle range of hunting purposes] it would probably be something in the neighborhood of the 6.5 x 55 or the .260 (really first known as the .263Waters Express and proposed sometime before 1960 by Kenneth Waters). I'm not sure I'd call the penetration from the .264 calibers to be "magic" but the penetration is way out of proportion for the bore diameter.
' "Deep penetrating bullets of good sectional density are propelled at comparitively modest velocities.' " (Finn Aagard on Hunting Rifles and Cartridges).
I hunt deer (mostly) and have aspirations of smaller game and (someday) larger game(Elk, Bear.....??) And although I would have a larger firearm with me for the larger game and a smaller firearm for the smaller game, ...I would be ultimately comfortable with the 6.5 x 55 as a back up rifle in applications from prairie dog to moose and elk. Mind you, I consider those extremes to be the outer edge of the 6.5's application, and perfoect conditions would have to exist before I'd take the shot with the back-up rifle.
I have had a love affair with the 6.5 caliber for over 30 years now, and while I don't believe it to be an "all-purpose" round,....it is MY favorite "Multi-purpose" round.
Those who regulary hunt the beasts larger than the whitetail would, understandably, prefer a larger "multi-purpose" caliber. This is just one man's opinion of his own favorite. Opinions......everybody's got one.