By default setting, Fifa 11 has made 4-3-3 formation for AC Milan. The formation is very deadly in ball possession and also attack. However, the 4-3-3 scheme is also good in defend. Then, what's the best formation for AC Milan today in FIFA 2011? I Think 4-3-3 is the best. You can play Zlatan Ibrahimovic alongside with Robinho and Pato as the strikers. In the midfield line, there K.P. Boateng, Pirlo, Ronaldinho or Ambrossini. And in the back line, you can use Zambrotta, Thiago Silva, Abate, and Antonini. Very solid formation at all for AC Milan.
Then, you can try your experiment in the formation, switch between one player with others so will result the best in your taste and style. By the way, AC Milan inhibited by many top players squad like: Abbiati, Zambrotta, Nesta, Thiago Silva, Antonini, Flamini, Pirlo, Ambrossini, Pato, Ibrahimovic, Ronaldinho, Robinho, Seedorf, Boateng, Gattuso, Inzaghi, Oddo, Yepes, Abate, Jankulovski, and more. They are the combination young and veteran players. If one player is injured, you easily find other players to replace him in AC Milan squad.