I feel like the 4-3-3 or a 4-1-2-3 - both narrow - is the perfect formation. The four defenders are usually more than enough to deal with threats from the opposition, and the two wing backs provide width. There are three midfielders in midfield, one of which is more defensive than the other two and helps shield the defense. Three midfielders is usually plenty to control the midfield, since most formations have two or three midfielders, so the worst they'll face is an even fight numerically. And the two inside forwards and one striker or three strikers can usually overload the defense, as the majority of formations run four defenders, or two central defenders and two wing backs, meaning that the attackers will outnumber the defenders a lot of the time, especially if counterattacking and the wingbacks (both teams) have pushed up.

Mind you, for this formation to work, the wing backs would have their work cut out, since they'd need to help out the attack while also defending well - but that's no more than most teams today demand from wing backs anyway. Also, the holding or defensive midfielder role is probably the most important on the team, so that would require a very good player.

Overall though, I feel like the 4-1-2-3/4-3-3 is the most offensively strong formation, while still being no more defensively weak than most formations. If the players can fulfill their roles, I can't see any real weaknesses in the formation.