There were many significant and important rulers in medieval Europe. It's hard to pinpoint a few which I think were the most significant ones, but without a doubt, William the Conqueror is one of them, as he was the ruler of England for a time and brought Norman rule over the isles (they were previously under Anglo-Saxon rule). Another important ruler would be Richard the Lionheart, whose attempted conquest of the middle east marked an important turning stone for the Crusades in the future.