Building construction order.

What additional (special) buildings are best to build for each faction?

In my opinion, the special buildings of Necropolis are the most overpowered, as they allow you to buy back fallen units for the first 3 days. Inferno is in second place with its meteor shower that damages the attacking opponent's castle. The rest are more or less equal. However, it's up to you to decide which ones are best to build. In my opinion, it's not always possible to build these buildings in the first month of the game, and you might be able to defeat your opponent by the second week.