Obviously there isn't a "best formation". But the formation i like to use is 4-5-1v after seeing some success from other high profile managers on here (dv8r i.e).

Before that i was using 4-3n-3, where i was getting a lot of goals, but also conceding a lot against flanked teams. Since switching to 4-5-1v, i've beat my rival in the league multiple times in the past 2 seasons who also uses 4-3-3. And my defense has been a lot more solid with switching my DMC to blue arrow if needed.