I'd opt for the Spartans too. They fought to the last man in Thermopyhlae.

Without question the Mongols. They were quite simply unstoppable by any army of their time. From Central Europe to Vietnam they swept all before them.

I would also like to "nominate" the Gurkhas. The mere fact that the British army (and even the Indian army) still maintains such a unit to this day is testament of the quality of men Nepal provides. Thry may be small in stature but big on discipline, tenacity and courage in battle which earned the respect of the British.

I'm going to go back 140 years to the late War of the Rebellion, and submit that The Iron Brigade was the finest fighting force in history. Consisting of the 2nd Wisconsin, 6th Wisconsin, 7th Wisconsin, 19th Indiana, and from October of 1862, the 24th Michigan, they were all volunteers, ordinary American citizens, defending the ideal of The American Experiment. This one brigade fought "Stonewall" Jackson's division to a standstill at Gainesville, assaulted and took a nearly impregnable defensive position at Turner's Gap at South Mountain, took the Cornfield at Antietam, retreating only when support did not arrive, and assaulted MacPherson's Ridge at Gettysburg, capturing a general and holding firm all afternoon until the collapse of the XI Corps rendered their position hopeless.

-Charlie