Len senses two monsters. He takes out his Advent Deck... and then puts it away. The next scene is the monsters running into... an untransformed Len. Who proceeds to own them. It's also a prime example of Tranquil Fury - Torque had convinced Kit that Len was the bad guy, and so Len was just generally having a rotten day, and - going suitless made the fight better for blowing off steam. He's mostly silent and emotionless for the entire fight.
Episode 17: The Power Of Three
This is more personal than anything else, but the fact that they managed to slip a West Side Story reference into an episode and not loose any momentum. ("You're depraved on account of you're deprived?")
The newest bad Riders haul out their Transformation Trinkets... and Len snatches them out of their hands before they can use them, and proceeds to spend half the episode singlehandedly kicking their asses.
What makes it even better is what Len says after snatching the decks: "I know exactly who you are...you're amateurs."
Episode 19: Semper Fi
Sting's sacrifice, and his final, quiet "Semper fi" with his hand over his chest as his last words.
this only gets stronger when you've seen the equivalent in Ryuki, when Ryuki/Dragon Knight was the one saved, not Knight/Wing Knight.
Episode 21: Strike's Ultimatum
Strike is the second-strongest Rider after Wrath, with a higher body count to his name than any other. Axe is quite strong, and he and Spear combined have proven stronger than Dragon Knight more than once. So when Kit must face all three together with no help, what happens? He wins using his brains.
Not to mention tops it all off by viciously venting Spear, avenging Sting in the process.
Also, Strike actually vented/killed more Riders than his counterpart Ouja.note Strike got 5 (Torque, Thrust, Sting, Siren, Axe); Ouja got 4 (Gai, Raia, Imperer, Goro!Zolda). In addition, Strike manages to vent two of the three Riders that Asakura wanted to kill the most but he failed to (Tiger/Axe, and Kitaoka!Zolda/Torque since there's only one Earthling as Torque in KRDK) and by irony vented the Rider who killed Ouja in Ryuki Episode Final (Femme/Siren).
Episode 28: A Dragon Caged
A non-combat but no less awesome one: Kit's escape from The Men in Black. Len and Kase ride to the rescue... and find out it's unnecessary. Kit escapes by asking for a glass of water (they use a plastic cup, not being stupid.) So he gestures while talking, "accidentally" spills it, then turns over the table partially, making it an obstacle to the agents as well as letting his Advent Deck slide down. He reaches beneath it to catch it, and by the time they get around the table, all they see is... his reflection in the puddle of water. In Ventara, Kit thanks them for the drink and walks away.
Episode 31: Xaviax' Wrath
Villainous one: Wrath vs. two Survive Modes. It ends with Len on his ass, and Kit... well.
Episode 32: Advent Master Returns
A black-clad Maya's debut as Siren.
Episode 33: Out Of the Void
The entire battle between Adam and Len, from the producers of both this series and Kamen Rider Ryuki (there was some Ryuki footage and some original. All awesome.)