Spectral Soda is the most valuable item in the game, due to how rare it is. It is a bottle that when used in battle, will give you the Spectral buff for two turns. This will allow you to dodge any attack for those two turns. After the buff is over, you will not be able to use another Spectral Soda for 3 turns.
The reason Spectral Soda is so valuable is because of how it's obtained. Rather than buying it or finding it in a chest, you have to use the Shoplift ability on a Dweller to find this. Dwellers are an advanced form of the Thief only found in Overgrown Grotto when you have a specific rating. Not many people have this item, because of how difficult it is to obtain or simply because few people know about it.