The go to caliber could change with the times, right now i think it would have to be something plentiful or something you have a crap load of already.
For me it would have to be a 308, a 22 rifle and a 12 or 20 gauge shotgun. Maybe a crossbow for when things need to be quiet.
The odd calibers like the 6.5's would be nice as i really like them but ammo is not plentiful and it is much more expensive.