First of all it is important to understand there is no such thing as a best tactic, there are one or more tactics that will be best in a given situation, but it all depends on various issues. 
As such, each tactical decision you take will always be a tradeoff between one or more things. You gain something in one area, and sacrifice something in another area. You go from 4-4-2 to 3-4-3 you will score more goals and concede more as well. You go from 4-4-2 to 4-5-1 you gain more possession, but get less dangerous up front. It's all a matter of tradeoff. This is important to keep in mind once you judge your players and decide what tactics your team will use. 

Formations have gotten 3 variables that decide their strength. These variables are all compared to a normal 4-4-2. The variables are Defensive Bonus, Possession Bonus and Attacking Bonus. 

Example: 
A 4-5-1 will sacrifice attacking bonus to gain possession bonus. Why would you want to do this? Well maybe your strikers aren’t really good? Maybe the opponents goalkeeper is a monster? Maybe you’re weaker than the other team and want to try and keep the ball more? A lot of different factors can influence whether you want to make a tradeoff between bonusses. 

You gain defensive bonus by having more than 4 defenders. If you pull a central midfield down to a defensive midfield, you will also gain a defensive bonus, but lose a bit possession as he will not join the attack as much. If you play with 5 defenders you will gain a defensive bonus, but sacrifice either attacking bonus or possession bonus depending on where you pull the guy from. 

If all your players were equally good, you would want defensive bonusses against better teams, and attacking bonusses against worse teams.