One of the commenters over at The High Road forum (.us, not .org) asked the question, what's the best all-around caliber for North American game.
Four pages later the discussion is still going on. The consensus is that the best all-around caliber is the .30-06. It's hard to argue with that answer.
The .30-06 is one of the best, ever. It's been around since 1906 and it's still going strong. Every bolt-action manufacturer that I'm aware of chambers rifles for that cartridge and the ammo manufacturers commonly list it in the top three of cartridges sold in North America.
You can buy ammo anywhere, virtually at any big box store, any gun store, and almost any country convenience store. Ammo is easy to find. It's easy to reload for, too. Page after page of .30-06 recipes can be found in virtually any reloading manual. I've loaded it hot, for velocity. I've loaded it down (about 1800 fps) for plinking. The best cast bullet group I ever saw was shot with a 200 grain Lyman bullet and the bullet traveling about 1600 fps, out of a bone-stock Remington 700 in .30-06. That 5-shot group fell into 0.76 inches with a called flyer. If he hadn't flown that bullet, the whole group would have been under half an inch. I didn't shoot it, but I saw it and it was the damndest exhibition of cast bullet shooting I'd ever seen.
By best target from my Savage goes into 3/4th of an inch. This with a 168 grain bullet traveling 2800 fps. Don't let anyone tell you that the .30-06 isn't capable of fine accuracy.
If you're hunting game up to the size of elk, the .30-06 will do the job. If you're hunting whitetail deer, the .30-06 will do the job. If you want one rifle that'll kill nearly everything in North America, the .30-06 is your medicine.
Every cartridge has its fan club. Some like the .270, some like the .243. I love the .30-30 and think that it'll do anything a .30-06 will do inside of 100 yards. Truth be known, most game animals are killed inside 100 yards. There are others that think the 7mm Remington Magnum is the cat's meow for all game. If you want a nice, light carbine get the .30-30. If you want the full size bolt action rifle, get the .30-06. I love my .30-30s. I currently own two and have one on order.
The hunting season is upon us, and while I'll likely be carrying one of several rifles this year, I know that I can rely on my .30-06. It's not flashy, it's not a whiz-bang magnum, it's not the latest, but it's likely the greatest.
Folks ask, what caliber should I buy if I can only buy one caliber. Either the .30-30, or the .30-06. I've never felt undergunned with either of them. But, if you're looking for best all-round, the .30-06 is the cartridge.