The first battle against the samurai, where Algren fights off four samurai at once whilst wounded, using an unfamiliar weapon (a Japanese yari). No wonder Katsumoto was impressed. Special emphasis goes to killing the samurai who was about to execute him by stabbing the samurai through the throat with the broken spear.
The first battle itself was awesome. The battle cries reverberating throughout the forest and the Samurai emerging from the fog on horseback was also just a moment of awesome.
Those poor hapless conscripts who got slaughtered en masse? The ones Algren rightly said weren't ready? Come the final battle they are, and they give the samurai a run for their money in a fight. No matter where your sympathies lie, it's pretty much impossible not to cheer for them.
The duel in the rain was impressive. Algren gets the living snot smacked out of him four times by one of the samurai with a wooden sword and keeps getting back up. It's at this point that the opinion the samurai have of him starts to change.
Algren fighting to a draw with Ujio.
Algren's return to Tokyo with Katsumoto and the samurai. Everyone on the streets immediately cleared a path for them and nearly all countrymen bowed deeply. Regardless if this was out of fear or respect, the significance of the samurai cannot be denied.
Algren's duel against the five ronin. Gets played back in slow motion so you can see how cool he was.
It's not just played for coolness. He's actively replaying the fight in his head to ask himself the question, "Did I miss anything?" Sure enough, Omura's personal servant survived the initial round, but Algren was still fresh enough to finish him off.
Nobutada’s Heroic Sacrifice. Having already been mortally wounded via a bullet in the retreat, he opts to stay behind so that Katsumoto, Algren and the other Samurai can escape the Imperial Guards. After a final heartfelt goodbye between father and son, he heads back into the fray and begins dropping Imperial Soldiers with his archery skills. After running out of arrows, he draws his swords and charges headlong into a hail of bullets in a valiant last ditch attempt at a Taking You with Me. He doesn’t make it, but his death is one that any Samurai would wish for.
During the final battle, Ujio enters by riding in with the mounted samurai and gets his horse shot out from under him. He executes a perfect dismount/roll and comes back up just carving through every enemy in his way.
He also casually snaps the neck of a soldier with a single smooth motion, without stopping his forward momentum.
Later on, he actually gets shot in the stomach, pushes a guy away who was trying to help him up, spits out some blood, and gets to his feet to continue the battle.
The final charge of the samurai. In spite of their valiant effort, the rebels tear through the Imperial force and gallop towards the cannon line. All points to the impending disaster for the loyalist side. And in the last moment, they break out the ace up their sleeve, a weapon not yet tested in Japan: Gatling guns. No matter their pride, skill at wielding obsolete weapons, or Hollywood on their side, the samurai fall like flies. Battle is over. (What?!)
The scene where the Japanese officer orders the Gatling guns to stop firing despite Omura yelling at him to massacre every last samurai is epic as well, if only for the guy having the balls to disobey a direct order from a technical superior, who up until that point had the Emperor under his influence.
The scene following said final charge, where the entire Imperial army kneels down out of respect to the fallen samurai with quite apparent expressions of sorrow is awesome in its symbolism and sheer heartstring-tugging beauty.
