Which two kinds of power have been found to be related most strongly to leader effectiveness in motivating subordinates?

These are "expert and referent power".

Referent power in authority is the capacity of a pioneer to develop the regard and deference of his supporters in such ways that they wish to resemble him. In short, referent power is showing others how it’s done. While expert power depends on workers' observation that a chief or some other individual from an association has a high state of learning or a specific arrangement of aptitudes that different representatives or individuals from the association don't have.