The Roma coach famously won the Premier League title as Leicester City manager in the 2016/17 season.
Mourinho was asked by former Chelsea midfielder John Obi Mikel about who was the better manager - Ferguson or Guardiola - the Fener coach went with the Italian.
"They are both historic coaches in the Premier League, but the most legendary for me is Ranieri," he replied.
"Winning 5 or 6 trophies with Manchester United or Manchester City in this generation is a great achievement, but leading Leicester to the title is something unique. There are no 'buts': it was the most incredible trophy I have ever seen and experienced."