Determining the most successful football club in the world is subjective and can depend on a variety of factors, such as the number of trophies won, the level of competition, the club's history and tradition, and its fan base. However, if we consider the number of major trophies won, including domestic league titles, continental championships, and international competitions, then Real Madrid is often considered the most successful football club in the world.
As of my knowledge cutoff date of 2021, Real Madrid has won a record 13 UEFA Champions League titles, 34 La Liga titles, 19 Copa del Rey titles, and numerous other domestic and international trophies. They have a rich history and tradition, having been founded in 1902 and featuring some of the greatest players in football history, such as Alfredo Di Stefano, Ferenc Puskas, Zinedine Zidane, Cristiano Ronaldo, and many others. Additionally, Real Madrid has one of the largest fan bases in the world and is one of the most valuable football clubs according to Forbes' annual ranking.