For me british is the best nation for attack until atomic age because:
- The British special unit is beyond the range of castles, redoubts and ballista/cannon towers/anti-tank guns because of its added range. It's extended range also means that the castle won't spawn any generals.
- It can attack deeply through enemy walls. This also makes it an excellent troop donation so it can safely attack enemy infantry behind walls.
The +1 range is the key and if you use it with a good build you will be deadly in your attacks.
For late atomic age and Cold War age the nation with the best attack is Germany because +15% damage bonus. since this is when the factory troops come into play (I think that players should not use factory troops until they are in the atomic age). so the 15% dmg bonus help factory troops.
and yes! the best nation of Cold War age is germany.