I would choose an army of Slivers, from Magic: The Gathering.
Each one gains the powers of all the others, so the total power of my army increases non-linearly with increased numbers. One sliver is worth one sliver, but two slivers are actually worth four, three are worth nine, and so on and so forth.
Additionally, they operate on a hive mind, with the individual soldiers having no souls to steal, no independent thought to disobey, and no fear for their own continued existence.
They employ Amoeboid Changelings that can turn their enemies into Slivers, and the Sliver Overlord can take control of any Slivers I don't already control.
In fact, I could have an assimilation race with the earlier poster who chose the Borg, and see whose army can assimilate the known universe faster.