Final answer:
As of December 31, 2000, Jean Beliveau had won the most Stanley Cups with a total of 10 as a player. He is followed by others such as Wayne Gretzky and Mark Messier, but Beliveau held the record for the most Stanley Cup victories at that time.
Step-by-step explanation:
The individual who had won the most Stanley Cups at the end of the 20th century, specifically on 31 December 2000, is Jean Beliveau. Jean Beliveau, a former Canadian professional ice hockey player, played for the Montreal Canadiens of the National Hockey League (NHL). He won a total of 10 Stanley Cups as a player with this team, the most for any player at that time. Notably, he also won 7 more as an executive with the Canadiens, making his total 17 Stanley Cups. The other individuals listed, Wayne Gretzky, Mark Messier, and Scotty Bowman, also have a significant number of Stanley Cup victories, but none match Beliveau's record as a player by the end of the 20th century.