Sitting Bull (Tatanka Iyotake) was pretty good. He fought against a powerful army that had recently won a major civil war and was victorious on a number of occasions. The Great Sioux War lasted 9 years, from 1868 to 1877 and perhaps Sitting Bull's greatest victory was the battle of Little Big Horn. It is definitely the most well known.

Or Geronimo (Goyahkla) would have to be right up there, but he was probably more the equivalent of a colonel than a general, as he never claimed to be a chief. Geronimo's period of military endeavour against two invading forces (Mexico and the white settlers) lasted for 28 years from 1858 to 1886 and he is renowned for his daring exploits and numerous escapes from capture. Toward the end of his military career he led a small band of 36 men, women and children and they evaded thousands of Mexican and American troops for over a year.

Both of these men would have to rate very highly on any list of great military leaders as they led forces that were outnumbered and poorly equipped in comparison to their enemies. Both failed to stem the tide of invasion but they both offered stiff resistance that caused many headaches for those invaders.

I know that they were not U.S. Generals and in fact were enemies of the U.S. Government. They were however great Americans who fought for the best of reasons - to try to stop invaders from taking their lands.