It's Reigning Monarchs
There have been many monarchs of many different states throughout history. Here are the 25 who have reigned the longest—with the only caveat being we will only be looking at monarchs of states who were internationally recognized as sovereign for most/all of their reign.
25: Conrad I
State: Burgundy
Reign: July 12, 937 - October 19, 993 (56 years, 99 days)
Known as "the Peaceful", Conrad I reigned from 937 until his passing in 993.
24: James VI And I
State: Scotland
Reign: July 24, 1567 - March 27, 1625 (57 years, 246 days)
He began his reign as James VI, King of Scotland in 1567—and in 1603, he became James I, King of England and Ireland, following the union of the Scottish and English crowns.
23: Wilhelmina
State: Netherlands
Reign: November 23,1890 - September 4,1948 (57 years, 286 days)
She was Queen of the Netherlands—and her more than 57-year reign makes her the longest-reigning female monarch outside the UK.
22: Nicholas I
State: Principality of Montenegro (1860–1910), Kingdom of Montenegro (1910–1918)
Reign: August 13,1860 - November 26, 1918 (58 years, 105 days)
Nicholas I was the last monarch of Montenegro and the principality's first and only king.
21: Al-Mustansir Billah
State: Fatimid Caliphate (Egypt)
Reign: June 13, 1036 - December 29, 1094 (58 years, 199 days)
Billah was the eighth Fatimid Caliph.
20: Pedro II
State: Brazil
Reign: April 7, 1831 - November 15,1889 (58 years, 222 days)
Pedro II (the Magnanimous) was the last monarch of the Empire of Brazil.
19: Louis XV
State: France
Reign: September 1,1715 - May 10, 1774 (58 years, 251 days)
Louis XV succeeded his great-grandfather Louis XIV when he was five years old. However, until he reached maturity (13 years old) in 1723, the kingdom was ruled by his grand-uncle.
18: George III
State: Great Britain, Ireland, United Kingdom
Reign: October 25, 1760 - January 29,1820 (59 years, 96 days)
In the later years of his life, George had some mental illness. The exact nature of which is not known for sure, but historians and medical experts tend to think that the symptoms and behaviors were consistent with possibly bipolar disorder or porphyria.
17: Christian IV
State: Denmark–Norway
Reign: April 4, 1588 - February 28, 1648 (59 years, 330 days)
This King of Norway's reign was the longest in Scandinavian history.
16: Qianlong Emperor
State: China
Reign: October 18, 1735 - February 9, 1796 (60 years, 114 days)
While his official reign as Emperor ended with his abdication in 1796, he kept ultimate power until his passing three years later, in 1799.
15: Kʼakʼ Tiliw Chan Yopaat
State: Quiriguá (Guatemala)
Reign: December 29, 724 - July 27, 785 (60 years, 210 days)
The leader of this ancient Mayan city-state was known by many names, including: Cauac Sky, Kawak Sky, Butsʼ Tiliw, and Butzʼ Tiʼliw.
14: Itzamnaaj Bahlam III
State: Yaxchilan (Mexico)
Reign: October 20, 681 - June 15, 742 (60 years, 238 days)
Itzamnaaj Bahlam III ruled the city of Yaxchilan (located in what is now Chiapas, Mexico) as Mayan King. He ordered the construction of many buildings during his reign—many of which are still there.
13: Honoré III
State: Monaco
Reign: December 29, 1731 - January 13, 1793 (61 years, 15 days)
Honoré III ruled for over 61 years as the Prince of Monaco.
12: Kangxi Emperor
State: China
Reign: February 5, 1661 - December 20, 1722 (61 years, 318 days)
The longest-reigning emperor in Chinese history.
11: Hirohito (Emperor Shōwa)
State: Japan
Reign: December 25, 1926 - January 7, 1989 (62 years, 13 days)
He was involved in the decision and approval of Japan's attack on Pearl Harbor.
10: James I
State: Crown of Aragon
Reign: September 12, 1213 - July 27, 1276 (62 years, 319 days)
James I the Conqueror was King of Aragon, Count of Barcelona, and Lord of Montpellier for all 62+ years—and King of Majorca from 1231 to 1276, and King of Valencia from 1238 to 1276.
9: Victoria
State: United Kingdom
Reign: June 20, 1837 - January 22, 1901 (63 years, 216 days)
Queen Victoria's reign is known as the Victorian era.
8: Ferdinand III
State: Naoples, Sicily, Two Sicilies
Reign: October 6, 1759 - January 4, 1825 (65 years, 90 days)
Beginning in 1759, he was King of Naples as Ferdinand IV and King of Sicily as Ferdinand III—and although he was deposed two times, he kept coming back. In 1816, he began his reign as the King of the Two Sicilies—which he maintained until his death in 1825.
7: Chan Imix Kʼawiil
State: Copán (Honduras)
Reign: February 5, 628 - June 15, 695 (67 years, 130 days)
The 12th ruler of the Maya city state Copán, Chan Imix Kʼawiil was nicknamed "Smoke Jaguar".
6: Franz Joseph I
State: Austrian Empire, Austria-Hungary
Reign: December 2,1848 - November 21, 1916 (67 years, 355 days)
Franz Joseph I took over after his uncle Emperor Ferdinand I abdicated the throne—as part of a plan to end the Hungarian Revolution of 1848.
5: Kʼinich Janaabʼ Pakal I
State: Palenque (Mexico)
Reign: July 27, 615 - August 29, 683 (68 years, 33 days)
Ajaw ("Lord") of the Maya city-state of Palenque, Kʼinich Janaabʼ Pakal reigned longer than any monarch in the history of the Americas.
4: Johann II
State: Liechtenstein
Reign: November 12, 1858 - February 11, 1929 (70 years, 91 days)
Nicknamed "the Good", Johann II reigned as Prince of Liechtenstein for over 70 years.
3: Rama IX (Bhumibol Adulyadej)
State: Thailand
Reign: June 9, 1946 - October 13, 2016 (70 years, 126 days)
He was given the title of Bhumibol the Great after his death. His fortune was estimated to be around $30 billion.
2: Elizabeth II
State: United Kingdom
Reign: February 6, 1952 - September 8, 2022 (70 years, 214 days)
Queen Elizabeth II took the throne—following the death of her father—at the age of 25.
1: Louis XIV
State: France
Reign: May 14,1643 - September 1, 1715 (72 years, 110 days)
Louis XIV, aka "Louis the Great", was only five years old when he became King of France. However, he didn't begin his personal rule of France until 1661. Following the passing of his chief minister, Cardinal Mazarin, Louis announced that he would take over the job himself.
He maintained his rule until his death in 1715—thus making him the longest-reigning monarch in history.