[Rytek]

Any of the summons that ramp up the higher level you go are easy to mass. Ghosts, statues, undead horse.

I agree with the statement that it depends on the nation. It also depends on who you are fighting.

A few wights interspersed with some LA Atlantis troops can really give them some kick.

Summer lions or Winter wolves behind your Abyssian or LA Atlantis heavy infantry set to hold and attack can chase down runners that the normal heavy infantry would let get away. The auras they provide can turn a close fight as well.

Against certain enemies, summons you wouldnt normally consider become an option. As mentioned, claymen vs R'leh.
How about a horde of dispossesed to hold up a line of Giants while your mages pound them. Mechanical men become an option if you are facing an enemy using Elemental magic. And Spring Hawks are damn expensive to mass, but if you are fighting someone using alot of lighting, it may be an option.

The best bang for the buck in the late game is whatever your enemy is vulnerable to.