I myself am an adherent to the style of play introduced bu Cruyff called Total Football (you might have guessed) where evryone plays at virtually every position, covering for each other and stuff, so that the game becomes very elastic.
Total Football is technically the ideal formation but it's extremely hard to pull off successfully. You need to have well rounded players who are smart and able to play both in attack and defense. It's pretty tough to find such players nowadays who can compete with other world class players.
Of course every team has their own tactic to fit the type of players available to them. Personally, I like playing something like this:
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x----x----x----x
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x----x----x----x
... with the wingers pushing up high on the full backs and one of the two center mids having a bit more attacking freedom to support the striker.