There only is one movie that hits all the " authenticity buttons " and that's True Grit, Jeff Bridges. Clothing, hardware, firearms and modes of speech. Note the sparks, smoke and report of the pistols and rifles: that's research. Look at Lucky Ned Peppers boot as it sits on Matties neck...".thats authenticity. Does Authenticity make a great western? No, but tie it into a great story and you have Something Special.
Early John Ford movies are good too. The Treasure of the Sierra Madre has it also.