In his previous comment Ishmael ranked the Apache as the toughest western Indian warrior and the Comanche as second, noting that the latter had more manpower and resources.
I have read they originally came by the name Blackfeet by crossing burnt over plains grass, blacking the bottoms of the moccasins. The terrain they lived in was part of the Great Plains mixed in with the Rockies of Montana. The climate was very cold, reaching temperatures of 50 below zero in the winter.
The tribe acquired horses about 1730, noting the advantages from observing Shoshone neighbors. Like the other plains tribes, this efficient way of hunting buffalo increased their range and territory and their ability to defend it. James mentioned how much they liked to kill whitey. I also read about it in the biography of John Colter, who had a foot race, naked, with male members of the tribe. They had already filled his trapping buddy John Potts full of arrows for refusing to bring his canoe to shore. Lewis and Clarke had pissed them off earlier on their return trip from the Pacific coast. The Blackfeet were reaching the height of their power, by the early 1800s, in my humble opinion they would have pushed the Sioux off some of their buffalo hunting areas, if smallpox had not decimated the tribe. Anybody who could push the Sioux around deserves the #3 slot.
There is a small tribe that successfully withstood Blackfeet through the years while living within their immediate proximity. They are called Tsuu T'ina (English name Sarcee) and have been respected for their toughness even by Blackfoot. Yet they are not of the same stock as Blackfoot. Their present day reserve is within the greater Blackfoot lands.
Had to add something about this but off track a little. The real question is how did Lewis and Clarke make it all the way across the country without being killed? Answer, they had an semi-automatic weapon. Yes! They said in their journals that whenever they met a tribe the first thing they did was display and fire the rifle. Beeman of Beeman rifles and many others believe that a rifle in Beemans possession is the exact rifle they used. It's a semi-auto air rifle. Remember I talked about high powered air rifles. Well this was one. Beeman has done the country a great service by saving this rifle and doing a tremendous amount of research that must have cost him plenty. He even made copies and engineering drawing. I have a book he published of the drawings.
en.wikipedia.org/wiki/Girandoni_air_rifle
To the Indians this must have been great magic to shoot 20 times in a row without loading. They could see that there were many other rifles in their luggage. Of course they couldn't know they only had the one with multiple fire but it worked.
I don't doubt the Girondi was impressive but ancient archers had a high rate of fire and deadly accuracy, similar to this:
youtube.com/watch?v=BEG-ly9tQGk
The Girondi shot lead balls which start to curve after too many yard and it took 1500 hand pumps to recharge. I'd prefer a bow to a Girondi any day.