For use in North America, the best overall cartridge is the .30-06 which will take down any large game on the continent with a well placed shot. Factory ammunition is available vitually anywhere if your silly enough not to reload. The .30-06 really shows its versatility if you reload, it has over 60 plus different projectiles available for reloading. This way your rifle can be used for anything from Varmints to Large game efficiently. Most people that buy a larger caliber are just trying to get down range power to make up for piss poor marksmanship and lack of pre season practice.
If you reload you already know that .308 diameter bullets used in the .30-06 are interchangeable in the following calibers;
1) .300 Win Mag
2) .30-30
3) .300 Savage
4) .308 Win
5) .30-40 Krag
6) .300 H&H Magnum
7) .308 Norma Magnum
8) .300 Weatherby Magnum
9) 7.62mmX 54R (slug barrel first to verify)
I shot my bragging shot with a Remington M788 in .308 Win fitted with a Bushnel/Banner 4x scope fitted with a BDC. I was using Remington 150 grain Core Lokt. I was hunting on the prairies and only able to get within 450 yds. I got into the prone, snuggled up to the stock, dialed into 450 yards and gently squeezed. 6 pointer mulie buck, 1 shot, heart shot! He went straight down. I never started out hunting season unless I had fired a minimum of 200 rds in practice for that year. Practice, Practice practice.