Answer:
Assocation Football Club?
Most Sucessful Domestic Club Team in the World
The most sucessful "professional" assocation football club in the World - [in terms of recognised national league championship titles] - is Rangers F.C of Glasgow. The team has won 54 league titles as of 2011/11 season. 27 league cups and 33 national cups. However, they have only won 1 European cup.
Most Sucessful International Club Team in the World
Real Madrid was voted the best club of the 20th C but statistically Boca Juniors and Associazione Calcio Milan [A.C Milan] have jointly the best record for winning 18 confederation clubs. Real Madrid is the 4th most succesful with 15 international titles.
However; the best club in the World is your own no matter what pain they put you through.