Any 30 caliber should work fine. A .270 is also quite good enough. Unless you have a specific need for a certain rifle think about ammo costs. If you're only going to have one rifle (for now) get one that has good ammo availability. 30-06, .308, .270, or even a .243 probably fit the bill. I got a .308 because my buddy had one and now we can share ammo. Not to mention that I can buy it everywhere.

As for shot placement, head shots are great if you can get close enough to be absolutely certain of a kill. As for me, when in doubt I always go for the shoulder. My theory is that it's better to waste some meat than to miss or wound the hog and have it run off. Something is better than nothing!

But back on topic, any 30 cal will work on hogs. As salukipv1 said, it might be nice to have a rifle that's good anywhere in the country. For that purpose, drop off the .243 and the 30-30. A .270 is adequate for anything in the lower 48 states, and a 30-06 or .308 is more than adequate.