For a rifle that you will take to battle, I tend to think the best caliber is the one that is most common, and one that you will be able to find. Common military/police calibers are a good choice. Of those you listed, the 5.56 fits the bill. Its not the most powerful, longest ranged, best ballistic coefficient, or any of that... but it is cheap and plentiful. You can accumulate a large stock of it now, and have the best chance of finding it in the future.
It never hurts to be able to resupply of the enemy, and to use his parts either, though you shouldn't count on being able to do that... and I guess that somewhat depends on what enemy you think you might have to go to battle against.
Personally I like the 7.62x51 for Second Amendment purposes because I like having a longer max effective range than those with intermediate calibers. I think the people who plan on going head to head at CQB range in a 2A situation are foolish. But it is generally good to have a rifle around in the most common service rifle caliber where you are, which for us would be 5.56. If you want to build a 5.56 rifle to use in a more battle rifle-like role, I would go with a 20" barrel to get all the velocity you can out of the round, and to have a longer sight radius for better precision. I would also go for one with 1/8 or 1/7 twist, so you can shoot the longer/heavier rounds that are more effective at long range. A light-profile barrel for light weight and chrome lining for long life and corrosion resistance are good ideas, too. Basically like a govt'-profile 20" upper from CMMG or BCM.