Always note if it is a “pure” defence, 
like “ONLY INFANTRY” – then send all infantry you got!
This can change during a fight!
The same goes for reinforcing a structure as then reinforcing a rally, more units of the right types is all that matters.
Movement speed is also good here, to get where you need to be as fast as possible.
To get to the structure that needs reinforcing:
City hop – drag and drop your troops from the sidebar to the next friendly city.
City hopping
It is possible to reinforce structures with multiple armies.
If one of your reinforcing armies get low on units, return them home and refresh.