1.) .308 Winchester
The 308 Winchester is probably the most accurate and efficient round ever devised. Compared to the .300 Win. Mag. or even the .30-06, for example, the amount of powder it burns is disproportionately less than the surprising velocities it produces from such a short case. Although some hunters believe it is underpowered, the nifty .308 is only 100 fps behind the .30-06. This lessens recoil, and in turn, enhances the shooter's ability to shoot it well. And because the military uses the "7.62 NATO" en masse, ammo can be had on the cheap. It has proven itself time and again in competition, in war and in the field.
2.) .22 Long Rifle
If I could have two rifles only, one of them would be a .22 LR. It is America's most popular cartridge, period, and for good reason. You can shoot all day for pennies, and for small game like squirrels, rabbits and vermin, there is nothing better. It's quiet, fun and good practice.
3.) .30-06 Springfield
Why is the Aught 6 not No. 1? It is so good at everything, that it is the best at nothing (except ruling retail sales and killing nazis.) If your buddy asks what caliber rifle he should buy, but he doesn't have a specific idea of what it will be used for, follow the Golden Rule and answer ".30-06." It's versatile, adequate, accurate and available anywhere in the world. If you don't have one by now you are weird, but there is hope.
4.) .375 H&H Magnum
If Africa is your dream, buy a .375 H&H and learn how to shoot it. You will be rewarded by only having to lug one rifle overseas. For the 4,000 ft.-lbs. of energy it can deliver, its recoil is not too bad, and its trajectory is flat enough to hold a maximum point-blank range of 250 yards. It is smooth-feeding, and not loaded hot enough to bind actions, yet it has the power to penetrate and kill the world's largest game, including African elephant.
5.) .300 Winchester Magnum
If I had to pick one rifle for all hunting, it would be chambered in .300 Win. Mag. Bullets of all types, weights and designs can be loaded in it; it can be downloaded to mimic the mild-recoiling .308 Win., or uploaded to speeds exceeding the .300 Weatherby. It is inherently accurate, and may be the perfect round for elk, black bears and Africa's plains game.
6.) .223 Remington
If it was not for the military contract on this hyper-velocity round, the nomination probably would have gone to a better performing, pure varmint round like the .22-250. But mass-manufacturing lowers prices and the .223 Rem., while fast enough and wonderfully accurate, is priced much better than its .22 centerfire competition. And since this almost recoil-less round is one that most normal NRA members want to shoot all day long, lower price means more fun.
7.) .270 Winchester
Little can be written here that wasn't already elaborated by Jack O'Connor and his .277-caliber-worshipping tribe. The .270 Win. is a slightly smaller, slightly faster, slightly milder-recoiling version of the venerable .30-06. It's fun to shoot, accurate, adequately powerful and it just plain works.
8.) .257 Weatherby Magnum
Perhaps the perfect round for big-country antelope and deer, the .257 Wby. Mag. did not earn the favoritism of the godfather of speed, Mr. Roy Weatherby, for nothing. The fastest of the commercially loaded .25 calibers-indeed, nearly all calibers-a .257 Wby. tipped with a modern, controlled-expansion bullet like a Barnes X hits distant animals with the speed and shock of a lightning bolt. It's also hell on prairie dogs.
9.) .416 Rigby The classical African round for dangerous game, the Rigby remains the high standard in ballistic performance-a 400-grain bullet at 2400 fps producing over 5,000 ft.-lbs. energy. Perhaps the all-time authority of such matters, John "Pondoro" Taylor, might dispute one point-he'd have the Rigby at No. 2. But then again, he hunted Jumbo like I do doves.
10.) 38 Winchester Magnum
Developed in 1958, the .338 Win. Mag. delivers a 225-grain bullet at 2780 fps, producing 3,921 ft.-lbs. of energy at the muzzle. This "Alaska insurance" has proven itself as a quick stopper of North America's largest and toothiest game.
The Grand Champion: 12-Gauge
Despite the naysayers who were weaned on rifles, the 12-gauge cartridge is the world's best. What other round can be used for pheasants, ducks and clay pigeons on the wing, turkeys on the ground or, with modern slug ammunition, deer, bears or anything else up to 150 yards? After you hunt all these things in a single fall day, bring it indoors, stash it under your bed and rest assured that whatever bad guy decides to enter your abode will probably not exit in one piece. Blessed be thy 12-gauge!
For more, check out John Zent's Top Ten.