potential value density...... the fuck. Well - an IOU could cover all of those and more, and there's potential for a fellow to honor his commitments even when doing so results in his death, so if we're going only for potential - that seems to be the obvious choice. Lowest potential? Plutonium or gold, I guess. Plutonium could be worth a lot, but certainly never more than food and water, except in cases where priorities are totally fucked, which is a proven possibility -- I guess plutonium can be thrown out, leaving gold, which is the only true luxury resource listed (ETA: that's not true, I guess - and, you know... maybe God would cut you a deal where if you bring him 6oz of gold, he'll give you his powers) -- but I could think of absurd scenarios where it's the only electricity-conducting element/alloy left, and you need to connect two devices together to save Earth or something... A human slave could be extremely valuable, since we're going for potential -- could be anybody...
I guess since we can make our own... worst would be... Idunno.... small quantities of dolomite. Best... being God?
You want to be God or use God for storing value? I'm not sure if you are serious about that. Actually, I chose things that have actually been historically considered highly negotiable. I almost considered obsidian, because it is somewhat scarce and I think it may one day again become highly sought, this time for scientific uses. But that's just my whimsy.
BTW, absurd scenarios are always worth considering if they have historical significance.