One would have to go back hundreds of years to find a monarch who reigned longer than Queen Elizabeth II.

In her 70 years on the throne, she helped modernize the monarchy across decades of enormous social change, royal marriages and births, and family scandals. For most Britons, she was the only monarch they had ever known.

Her death in September was arguably the most high-profile death this year, prompting a collective outpouring of grief and respect for her steady leadership as well as some criticism of the monarchy's role in colonialism. She likely met more people than anyone in history, an...