Answer: Pius IX is the answer. He started reigning as Pope on June 16, 1846 and died on February 7, 1878, and reign of 31.6 years. According to internet research, the average reign for a Pope is 7.5 years. See this internet article: Longest Reigning Popes

    
Pope Pius IX Issue, Scott # 632-634 (1978)

For more on Pope Pius IX see:
• Michael Lamothe Vatican Stamps - The Art of the Engraver - Artist, Pietro Arghittu Vatican Notes Volume: 68: 2020, Issue 383, Page 56
• Thomas K Adkins Pope Pius IX: Philatelic Retrospective Vatican Notes Volume: 55: 2007, Issue, 5 , Page 23