Currency should have, as stated before, little to no usefullness attached to it. The Value currency has is given by the trust of the people in said Currency, and it usually was backed by something else, like rare materials.
That said, essential Items like Water and Food make good BARTERING items because everyone needs them, but they’re bad as currency because the value would highly fluctuate, due to the ease of producing both. (Inflation, anyone?)
Ammunition, drugs and medicine would make sense as Currency because they are somewhat hard to reproduce properly and they are also somewhat rare - There are also so many different kinds of Ammo, meds and drugs that it would be an extremely complicated system for currency (is my 9mm bullet worth more or less than the .357? how about rare ammo, is it worth more? Or less, because nobody uses those guns?)
We also know that products like alcohol, cigarettes and similar commodities are used for trading in times of war ; This could potentially work out better than the aformentioned items, especially because they aren’t necessary to survive, but nice to have - at least if you drink or smoke. And I wouldn’t be surprised if most people in the Cataclysm started drinking, smoking or something similar to cope with everything around them.
If I have to be honest though, I think fallout did it right. They had several different currencies in their games, and they worked out well in my opinion:
- Bottlecaps. Hard to reproduce properly (as long as you don’t have the necessary machines), fakes can be somewhat easily spotted AND there is a high, but limited amount of them in the world.
- Ring pulls. Only used in Fallout Tactics, but it’s basically the same idea as the bottlecaps, but easier to reproduce, because they have an easier form and no paint on them.
- NCR-Dollar ; Legion-Coins, Scrips: Those are handed out by bigger factions (namely NCR, Legion, Brotherhood of Steel), and those fations will back up their respective money, so it’s at least somewhat safe because you know you can trade with this money at least in some locations. They never truly got rid of the bottlecap as secondary currency though.
With that said, it would make sense to use something similar as currency for catacylysm. Some kind of trash you usually wouldn’t really need in large quantities, preferably something with a low weight and volume, so you can carry a lot of it with you, if necessary. To be honest though, I wouldn’t really know what to use - Maybe Bullet Primers? We only have a few kinds of those, but they arent really…plentiful.