Ferdinand of Aragon
Also Known As: Ferdinand V
Ferdinand and his wife, Isabella of Castille, underwrote the voyages of Christopher Columbus and so established Spain as a powerful force in the New World. Their 1469 marriage was politically arranged as an effort to unite their two kingdoms, an effort which proved successful; under their rule Spain emerged as a unified and newly powerful country. Ferdinand and Isabella moved to establish Roman Catholicism as Spain's dominant religion, expelling Jews and Muslims and creating the infamous Spanish Inquisition. (Their efforts earned them the nickname Les Rois Catholiques, or the Catholic Monarchs.) Ferdinand oversaw multiple wars, most famously with the Moors of Grenada and with France in the so-called Italian Wars. Ferdinand outlived Isabella and upon his death in 1516 power passed to his grandson, Charles, who later became the Holy Roman Emperor Charles V.
Extra credit: Ferdinand and Isabella's youngest daughter, Catherine of Aragon, was the first wife of Henry VIII of England.