Name the monarchs who united Spain and made it a powerful nation?

Isabella of Castile-Leon and Ferdinand of Aragon defeated the last of the Moors, then evicted them and the Jews.

They DID NOT unite the three kingdoms. Isabella and her descendants remained the only monarchs of Castile-Leon and Ferdinand and his descendants were the only monarchs of Aragon.

Carlos I of Spain, their grandson, inherited all three kingdoms in 1516. Carlos I of Spain was also know as the Holy Roman Emperor Charles V. That was the genesis of modern Spain.