There is a lot of debate about the best all-around caliber for a defensive rifle. There are many good choices out here - 30-06, .270, .223, 7.62X39, 7.62X54R, and on and on. Along with many others, these are all good choices. I own some of them myself.
But, my choice for my primary firearm is the.308 Winchester (7.62X51). It has a lot going for it. Normally, it is amply available. Being a semi-current military caliber it is popular around the world. You should be able to find this caliber anywhere that has ammo for sale. You can buy imported low cost ammo for practicing and higher quality ammo for serious work. It functions fine in semi-autos. It is very accurate in most types of firearms. It is easy to reload. There is nothing to dislike about this caliber.
Having said that, I used to lean toward the 30-06. This caliber is the one that EVERYTHING is compared to (...itís almost as fast as the 06, itís a third again as fast as the 06, itís as powerful as the 06, and so it goes). And, most of the virtues of the .308 also apply to the 06. The 06 has been used for years with good results by military snipers. It has a lot going for it.
But, I recently watched a program on the History Channel where Marine snipers were target shooting at 1 mile with their .308 rifles! And, they were hitting their targets! I was impressed! So, even though the .308 doesnít have quite the power of the 06, I believe that it will be sufficient for my needs.