Very hard to beat the AK if choosing one rifle for defensive fighting, but better also have an M1A and an M4 so you can shoot whichever ammo you "acquisition" that day. And a Remington 700 in .308, and the .50 cal, and...
Homeland defense as in standing shoulder to shoulder with my fellow countrymen fighting a foreign invader, or homeland defense as defending my homestead?
Either way, AR-15. The main reason is that ammo, mags, and replacement parts are far more common than parts for ANY other MSR system will ever be. Even the AKM can't compete with that (ammo maybe).
I have to agree that if you can have more than one, a selection of calibers is always a good idea. And having a good deer rifle with a decent scope - preferably with a BDC reticule gives you a very good sniper rifle.
But I think I would recommend getting your choice of carbine and then a bolt gun and probably a good pistol in a common caliber before you get additional calibers or a shotgun... Just my preference. And of course, good web gear, extra mags and a sufficient amount of ammo - and I personally put that before additional caliber weapons.