What formation do you like? What makes it strong or weak? Any thoughts as well.

For me I have to go back to the old 4-1-2-1-2 from 6 years ago. It provies a lot of room for modification. Having 2 set strikers is quite the advantage while keeping a defensive mid back help prevent most attacks. The two mids can either be set out as central mids (flooding the midfield even further providing a pivotal point for defense and offense) or they can be set out wide as a right and left midfielder providing crosses to both strikers. 

4-2-3-1 and 4-3-3

3-5-2. demands hard working wingers in both offence and defense though..

I personally think that the 4-2-3-1 that Mourinho specializes in seems to be the most effective. People may argue that Guardiola's modified version of the 4-3-3 is more successful, but their team has been playing the same playing style for years and years, whereas Mourinho proved that his formation is acceptable and successful with multiple teams (Porto, Chelsea, Inter, Madrid).

The good thing about the 4-2-3-1 is that is allows you to counter-attack AND hold possession. The two holding midfielders (Alonso, Khedira) can distribute the ball out wide to the wingers (Ronaldo, di Maria) or to the creative playmaker (Ozil) sitting behind the striker (Benzema). They can repeat this process as many times as they like, as long as the opposition team isn't marking the two defensive midfielders. In turn, the striker gets continuous supply as he has the wingers whipping in crosses and the playmaker and defensive mids giving through balls.

The defensive unit is pretty stable too, as long as your full-backs aren't caught too far up the pitch (which is a HUGE problem for both Madrid and Barcelona, and it seems to be their key weakness). The positioning of the two defensive mids will allow them to back-track and intercept oncoming passes and make tackles without the center-backs having to get involved, and the full-backs can keep an eye on oncoming wingers. The CB's also receive a lot of protection from the defensive mids, which can lead to a less chance of them getting caught napping.

So to sum it all up, it's a great formation, but not a perfect one. All formations have their flaws. :P

4-3-3 is the best. 

4-4-1-1 for me. 

4-2-3-1 I think it's the most organized formation defensively and offensively 

4-2-3-1 and 3-5-2 and 4-2-2-2 ( baically a 4-4-2 but I think it's better with 2 CDM's and a 2 CAM's)

4-3-3 

# for a friendly match : 4-3-3 ( more entertaining football )
# for an official match : 4-2-3-1 ( more effective in both attack and defence )

@ knibis: Italy used that formation quite effectively in the first game of group C in Euro 2012. It really neutralized Spain's midfield. They could have won but Mario had a poor game

I wonder why they changed it to 4-1-3-2 in the final when it had worked so well the first time round.

But yeah, 3-5-2 is a favorite of mine. I also like 3-3-1-2-1, but only in video games ;)

emm...4-2-3-1,3-5-2,4-2-2-2.My favorite one of this is the 4-2-3-1.