Hers is among the most valuable collections ever assembled by a single person.
Does this mean that her collection is one of the best? or Best one (number 1 without competitors)?
The preposition among generally means being a member of a larger set. Therefore, it means that her collection is one of the most valuable collections of this kind. In other words, her collection is a particular member of the set of the most valuable collections ever assembled by a single person. There are many valuable collections like this. Hers is just one of them. Not the most valuable one and not the best one of them. Just one out of many similar ones.