Konekomaru's epic dive to save Izumo's birthday cake in episode 19.
Bon and Miwa using a Fatal Verse in Chapter 6. They knew that Fatal Verses for Ghouls usually came from the Book of John, and since they didn't know which, they instead recited the entire book. That's 21 chapters. And it worked too!
Episode 14: Bon creates and communicates an elaborate plan to cross a riverbed of demonic leeches while chanting.
Chapter 22: Rin, ofallpeople manages to pull a Stealth Hi/Bye in the middle of a room filled with exorcists. Not even Shura or Tatsuma realizes that he had change position until he grabbed Tatsuma's robe and started getting on Bon's case. It's subtle but it shows just how Badass Rin has the potential of becoming.
Chapter 30, has alot of these..
Shima coming back for Konekomaru after leaving him behind to run for himself.
Shiemi finally showing us that she has developed a back bone and orders the missing Ni-chan to come out and do something.
Yukio has been having trouble with Toudou for the longest time. What does Juuzou do? Smack his ass with a rod. That's what he does.
Chapter 33: Rin manages to control his flames enough to burn only the Impure King and every particle of it, leaving everyone and everything else untouched.
Chapter 38: Arthur—despite being a major tool—manages to take down the Impure Princess in a single shot.
Chapter 39: Mephisto Pheles reveals who he really is while showing just how outclassed Rin was so that he didn't get a big head after defeating the Impure King.
Chapter 41 has Izumo and Shiemi team up to take on a demon.
Chapter 43: Konekomaru grows a spine and calls everyone out on their flaws and advises them on what to do to defeat the family portrait shapeshifter.