Reading recent comments I had to add this.

Most underrated:
Elephants

Its true that this unit is fragile. Used by AI is almost no threat(unless You forgot Your missile units ).
You have to be carful how You use it and it has limited use.
For example I would never bring Elephants against celts.

But it is also only unit that routs enemy army in like 20 seconds.
It provides one sided victories that lets You to plow through enemy armies with minimal casualties.

I just miss them so much in my Haysadan campagne.
If I could get one unit of this beasts I would just devastate Ptolemies numerous armies.