D'Artagnan's growth throughout the series, which sees him just about able to take on two musketeers in the first episode...to taking on a beast like Lafarge and becoming a Musketeer due to his valour.
In the series finale, the Musketeers engineering an accidental public confession for the Cardinal, in which he implicates himself in the assassination attempt on the Queen.
Topped only by the Musketeers being Big Damn Heroes and going to rescue Constance, held hostage by Milady. It's the four of them versus a gang of season thieves and killers, and the bad guys do not stand a chance. You know it's awesome when the theme song is blaring throughout the sequence.
Constance taking out an armed baddie with a sword. She's only been taking lessons for a short time, she's holding the sword with both hands so she doesn't drop it, and yet she stabs him, disarms him, and smacks him over the head while Aramis sits behind her with his hands full.
In 2.02, D'Artagnan and King Louis have been captured by a gang to be sold to Spain as galley slaves. The man responsible has no idea who Louis is, and finally demands the truth, otherwise he will shoot D'Artagnan. Cue Louis calmly standing up tall and staring the criminal down.
King Louis: I am Louis, son of Henry IV of the House of Bourbon and Marie de Medici. I am your king, and you cannot treat me this way! *punches Sebastian in the face*
And from the same episode, we have a villainous one for Milady de Winter. She goes from being Sebastian's lover/accomplice (which is several steps down from when she was working for Cardinal Richelieu) to gaining the King's favour (and lust) by saving his life and playing the victim.
D'Artagnan calmly and implacably facing off against Gus (the boss of the whole operation) alone with Guscharging him down with his sword drawn on horseback and D'Artagnan, on foot, "armed" only with Athos' borrowed scarf. The youngest Musketeer then uses said scarf to grab said sword, throw the rider to the ground and stab the sonuvabitch when he's down.
Constance sneaking the deathly ill Dauphin out of the palace at night so she can let him sleep in a public laundry place, to let the steam clear his lungs so his fever can break. She nearly gets executed when she and the baby are found, but fortunately the cure works and the Dauphin's life is saved thanks to her quick thinking.
Spanish general Alamand pulling a Taking You with Me on his racist former lieutenant, killing them both and destroying the formula for the white gunpowder forever.
A villainous one for Rochefort in 2.04. He finds out who Milady is and pays her to assassinate the Spanish ambassador who's been conspiring with him (and who is on the verge of telling the King everything) while under the Musketeers' protection. This costs Treville his job as Captain, this removing the only other person in King Louis' inner circle that can still influence him.
Treville in 2.05, showing everyone how he earned the title of Captain even though he's no longer in command, by convincing the villagers of Pinon to fight for their land, training them into a semi-acceptable militia and giving them orders during the siege.
Milady escapes Marmion in 2.06, and immediately rides to the Musketeer garrison to warn them that the King is being held hostage. And then she personally helps in the mission, sneaking back into the chateau and dispatching a few baddies herself. Her reward for all this is to be summarily dismissed by Louis because she 'abandoned' him and was dressed like a man.
From the same episode, Constance is about to be shot by Marmion. Very calmly, she tells him to look her in the eye when he shoots her, and to think about how ashamed his family would be if they could see the kind of monster he's become in his quest for revenge. Even Marmion's brother agrees with her, telling him he's gone too far.
Aramis surviving a fall from a glass window, and climbs back up to rescue the Queen and the Dauphin. All while bleeding and having an injured leg.
Rochefort reminding everyone that he used to be a soldier before he became the King's minister, dispatching Marmion's thugs before gunning down the man himself. It's also a Call Back to the beginning of the episode, where Rochefort told Marmion that he would kill him himself.