The Bastion de La Rochelle. The four heroes (and their servants) go to a recently seized bastion to have a quiet place to discuss Milady's schemes and end up fighting off two waves of Hugenots attacking them. Thanks to a number of available muskets (and their servants to reload), they manage to gun down multiple enemies. It's such a moment that its spoken of reverently by the four and others even to the final book.
An understated one compared to the more action-heavy C Mo A in the series, but when Richelieu confronts the four for their murder of Milady, they produce Milady's blank check from the Cardinal himself, which says that the bearer of the letter was acting under the full orders and authority of Richelieu. The man is so impressed by it he commissions d'Artagnan an officer in the Musketeers.
Twenty Years After
Say what you will about Mordaunt, but he goes further with his Roaring Rampage of Revenge than most villains. He plays everyone to a hilt and kills the executioner of Lille, his uncle, King Charles I, survives a duel with d'Artagnan, and then almost manages to blow up the Musketeers while on a ship. When that fails, he still attempts (and nearly succeeds) at a Taking You with Me moment with Athos. He's not even 25.
The prison break of the Duke of Beaufort is one for both him and Grimaud. It runs on The Plan and Crazy Awesome.
The Musketeers themselves: They manage to outwit all of their opponents, negotiate an end to the Fronde rebellion, and force Mazarin to reward them for their efforts.
The entire defense of Belle Isle. With minimal help, Aramis and Porthos (in their mid-late 50s at this time) manage to fight off far more numerous soldiers, capture one who turns out to be the son of a minor enemy from the first book, befriend him, hold off even longer, and make an escape that prompts Porthos' above C Mo A.
D'Artagnan rebukes King Louis XIV for sending Athos to prison:
"Sire; it is for you to choose. Do you wish to have friends or lackeys - soldiers or slaves - great men or mere puppets? Do you wish men to serve you, or to bend and crouch before you? Do you wish men to love you, or to be afraid of you? If you prefer baseness, intrigue, cowardice, say so at once, sire, and we will leave you, - we who are the only individuals who are left, - nay, I will say more, the only models of the valor of former times; we who have done our duty, and have exceeded, perhaps, in courage and in merit, the men already great for posterity. Choose, sire! and that, too, without delay. Whatever relics remain to you of the great nobility, guard them with a jealous eye; you will never be deficient in courtiers. Delay not - and send me to the Bastile with my friend; for, if you did not know how to listen to the Comte de La Fère, whose voice is the sweetest and noblest in all the world when honor is the theme; if you do not know how to listen to d'Artagnan, the frankest and most honest voice of sincerity, you are a bad king, and to-morrow will be a poor king. And learn from me, sire, that bad kings are hated by their people, and poor kings are driven ignominiously away. That is what I had to say to you, sire; you were wrong to drive me to say it."
Of course, the all-powerful Sun King cannot resist the pure awesomeness of this speech and gives the order to set Athos free.
D'Artagnan gets at least two more: securing Charles II's position by kidnapping General Monk (right out of his camp) while disguised as a fisherman and his death scene.
Aramis springing the King's twin brother from the Bastille, putting Louis in his brother's cell and very nearly managing to put Philippe on the throne. Too bad Fouquet decided to put Honor Before Reason.