Ferdinand and Isabella ruled Aragon and Castile jointly from 1474 to 1504, a thirty-year period during which the modern country of Spain was born, ships they commissioned discovered and explored the New World, and the Spanish Empire was formed in the Western Hemisphere. Spain achieved its peak during the reigns of their next two successors (Charles and Phillip), who largely preserved and built upon the policies and institutions they founded. Each of them was already a monarch when they married, and during their marriage, they shared power over both kingdoms. In other words, it was not an instance of one person wearing the crown and the other merely being the spouse/consort. (One could argue over whether their powers were equal, but that's an argument for a different question. The fact is, they both had royal power.)
I want to fairly and accurately state Ferdinand and Isabella's importance relative to other royal couples. My question is, would it be correct to state that they were the most important royal couple in world history? In European history? In medieval European history? Or if "important" doesn't work, what about another adjective that does (successful, famous, influential, etc.)?
Just to stress the point, I'm uninterested in comparisons with royal couples where only one member was actually a monarch (regardless of how much behind-the-scenes influence his/her spouse is believe to have had).