I'm no gun nut but I've been looking into this too.

.22LR is cheapest and easiest to carry in quantities and is going to be my first choice.

For bigger game, I'm convinced that the best choice should be based on availability. The 3 most common sizes available in north america as far as I've been able to determine is 12 ga., .308, 30-06...not necessarily in that order. One of the local gun shops has a pallet of .308 boxes right in the middle of the floor ready to sell 250 rounds at a time and I'm sure there's many more in the back.

When the time comes to buy a large calibre rifle, I'll likely go with the shotgun...it's just too damned versatile especially if you get one with a second rifled barrel. I like to think my opinion is somewhat unbiased since I've never owned a gun before.