It's hard to qualify an unit as being the best at any role in KoW, because often a slightly "worse" but cheaper unit will still be able to do the job while freeing you enough points for something else.
It all depends on your general battle plan and how your units support each others.
You could say that a big def6+ unit is the best anvil, but if you are simply trying to hold a charge from a lone knight troop (or even a regiment), then maybe a simple regiment of spearmen would have been enough.
It is said that KoW is won during the movement phase, but this is also a matter of using the appropriate tool as often as possible while making your opponent waste ressources.
If you lost a war engine to a dragon hero, then it's bad, but you still had your opponent use a 300+ point unit to kill one worth less than 100, where a simple cheap mounted hero or light unit would have done the same job.
Hopefully this means that you had the advantage in points in other sections of the battle field, letting you get the advantage in maybe more importants local battles.
Of course, sometimes you really need a job done, and having a subobtimal unit do it it still better that not doing it at all, but it helps to be prepared with the right tools as often as possible.
So i you want to take the most expensive option for any one role, you have to make sure that you really exploit his potential most of the time compared to the cheaper alternatives.
Following this idea and to go back to your original topic, descriptions of each unit should probably include suggestions of how to best use them, usually by telling how they can support or be supported by other units.
I find that it is often a good idea to separate your army in several "detachments", small groups of one or more units, often made to be deployed around some inspiring hero
Of course you can have wildcards units that will be added where they will bring needed support, but even there it helps greatly to think of at least one or two default ways to include them.