The Immortals, one of the deadliest fighting forces in all of Ancient Asia. Spartan hoplites, and the Roman Legion was one impressive army.
One thing I thought was kinda cool about the Immortals is that they're one of the few actual Ghost armies. They're reported in Thermoplye, Marathon, and many other Greco-Persian wars, and other Persian conquests. They're even mentioned by Alexander the Great, some two hundred years later! But there is hardly anything known about them, and no one knows how they fought or how good they were. They were indeed the Immortals, because when they're mentioned in battles, the next paragraph will be Persian victory. Ephiates gives Xerxes the secret path, he sends the Immortals. Spartans loose within hours. They were defeated finally by Alexander the great, though. Few offical armies are so few, however. Ten thousand of them in the battle of Issus? Are you crazy? Alexander had hundreds of thousands of hoplites, and all other Persian forces were threefold that or more. Very cool, very odd army in my opinion. Who led them? No one knows. How they fought? No one knows. But they must have been effective to last for hundreds of years, and they never changed the amount once.