Kublai proving his worth as a leader when he faces off against his own brother to protect his empire.
The attack by the Hashashin, and the defense against it. While the palace guards are no match, Marco, Byamba and eventually Hundred Eyes all enter the fight, onebyone. It's the first time seeing them all in action, and it's fantastic.
The debut of Marco's trebuchets, lobbing stones en masse on the walled city.
The battle to take over the walled city. Every single moment puts you on edge and awe. Jingim demonstrated how formidable a warrior he is instead of a useless prince covered by his parents like most people thought. He effortlessly cut through lines of enemies and nothing short of a gun could stop him. Byamba dominates everyone who gets in his way. And Byamba's last minute rescue of Jingim and Marco rushes to his aid then fights alongside Byamba.
The duel between Hundred Eyes and Jia Sidao. Everything about it, from the fact that the two previously unrivaled fighters have met their match, to the reveal that Hundred Eyes knows the Praying Mantis style, to the seemingly effortless coup de grace.
Mei Lin bluffing her way out of a violent fate for herself and Marco at the hands of the loyalist brigands. Likewise, Marco showing the trust in her to put himself in chains when Mei Lin is meant to be his captive.
Marco Polo and Mei Lin dueling with the unknown Song loyalist in their pursuit of the boy Emperor. Both of them are very competent fighters, and even working together, they still barely survive.
When Marco returns to Cambulac, his sparring session with Hundred Eyes is just as awesome. It's implied that Hundred Eyes was going easy on him in order to see what his fighting technique looks like (he floors Marco with a single strike once he recognizes the technique), but the actual exchange of strikes is incredibly intense, all the more because Marco is so obviously going up against the very toughest fighter in the kingdom.
In a villainous sense, Ahmad goading Kublai into killing Zhao Xian. He preys on Kublai's existing insecurities about being too sympathetic to the Chinese, and destabilizes Cambulac without incriminating himself in the slightest.
Jingim defeating a wrestling opponent by employing his own martial arts skills. While the moves are illegal for the match, disqualifying him from victory, it also spares him a humiliating defeat, and leaves him as the only one standing.
Marco managing to derail Ahmad's plan simply by being nice to Mei Lin. When Mei Lin realizes Ahmad has no intention of keeping his word about freeing Ling Ling (and even plans to sell her), she realizes that Marco has actually been consistently decent to her (trusting her, giving a glowing report) to the point that when Marco enters Ahmad's tent looking for clues she agrees to tell all if he frees her daughter. He does, prompting her to confirm his suspicions. From there on Marco proceeds to tell Jingim, sending the plans of Gregory, Niccolo, Kaidu and Ahmad crashing into ruin.
The way Marco persuades Jingim of Ahmad's treachery. He simply says he was suspicious before Mei Lin confirmed it, and then recaps all of Ahmad's advice (and all the chaos that has come with it.), before pointing out that Ahmad was the boss of the guy who hired the assassins last season. He than asks Jingim to trust him, and says that only he can give the command. It works.
Byamba's rescue of both Marco and Jingim. Just the scene of these three finally working together makes you feel awesome
Kublai once again proves his worth as a leader after he learns of Ahmad's betrayal. After breaking the news to Chabi, he strides out of his tent, takes command of the situation, and in response to learning how outnumbered he is? A simple, deadpan yet completely collected: "We're going to have to improvise."
The entire battle scene in Heirs, starting with Kublai's "improvised" attack (which is both this and rather Nightmare Fuel), and leading to the long sequence of named characters proving their worth in the fray. Besides the Song loyalists, practically everyone in the show who can fight is in that one encounter. And it is glorious and hellish all at once.
While established Badasses like Khutulun, Orus, Byamba do well during the battle as expected, Niccolò Marco killing several Mongol warriors easily with a sword isn't and it's awesome.
Crusader Versus Shaolin Monk: Hundred Eyes takes on a heavily armoured Crusader whose chainmail renders him impervious to slashes. The monk has to settle for crushing the warrior's throat with two fingers. Short but awesome fight.
Jingim's speech as he volunteers to retake Cambulac in his father's name, finally making the crucial step to reach out and take his role as future Khan. The speech continues through intercut shots of Kokachin giving birth to his own heir, showing that as he accepts his role, he's bringing the path of succession full circle.
The fight in the throne room, intercut with Kublai's fight with Kaidu. In both cases, a member of the Khan's bloodline has failed to defeat their enemy and is about to be killed, only to be saved by the efforts of the people who support them. Even better, in the throne room, this is possible only because of General Qaban's Heel Realization. A minor character, who started out harassing Byamba and then supporting Ahmad, ends up fighting at Byamba's side against Ahmad's guards when it comes to light that the Khan has been betrayed. The entire sequence is the perfect testament of the loyalty Kublai earns of his subjects.