I would think the most valuable ammo would be whatever your best gun uses. In terms of just general value, I would think that other things such as food will have more value than ammo. So if you're thinking of collecting something of value just in case things go bad, I would recommend food stores and seeds. Seeds are a lot cheaper than ammo and will produce something that will be more valuable. Of course if you had something as valuable as food if things ever went really bad for an extended period, that ammo for your best gun would probably be more valuable to you than anything else.
Just my .02