Zatch Bell!/Awesome

The end of the Faudo arc contains one for just about everyone- Ponygon, Kanchome, and Tia all fight bravely against Rodeux despite getting beaten. Momon alerts everyone that Kiyo is coming back, which gives him the willpower to successfully fend off Zeon's two Dragons while carrying Zatch on his back, without a single offensive spell. Then he sacrifices his book to save his partner, and tearfully apologizes for failing everyone- except Kiyo returns earlier than he expected to tell Momon that he did a great job just before he disappears. Then Kiyo and Zatch deliver the most epic curbstomp battle in the series to Rodeux, having already mastered multiple new spells that he's never used before. Then there's the whole battle between Zatch and Zeon, with the debut of the true Bao, everyone helping Zatch overcome its evil spirit, Cherish overcoming her fears and defying Zeon, and finally after Zeon's defeat and redemption- his giving Zatch his electrical powers to boost Bao even further and destroy Faudo in one shot before it destroys Japan.
Brago's short but wonderful the reason you suck speech to Zofis in episode 88. I don't think I was the only one who assumed this from the look on Zofis' face.
Just about anytime that Bao Zakeruga is used.
- Turned Up to Eleven when it reached it's true form.
The end of the Clear Note battle when all of the previously defeated Mamodo and Brago combine their powers with Gash and Kiyomaro's Golden Book
Kid instructing Kanchome and Falgore how to defeat Belguim E.O. after Professor Riddle had been gravely injured and while his book was burning.
Sherry and Brago defeating the group of thousand year Mamodo.
Umagon and Sunbeam during the fight with Clear Note.
Cherish and Nicole shooting at Zeon and releasing the captured Umagon, Tio, and Kanchome, while overcoming her trauma from Zeon's lightning.
Brago and Sherry's entire fight with Zofis and Koko
Gash when he used his own body as a means to hold up the barrier, knowing full when that it would kill him and then...
- Bari for taking over and allowing Gash to escape knowing full well that the barrier would kill him.
- In the same vein, the flashback to Bari taking on an extremely powerful dragon demon, and winning. The dragon is so impressed by this he happily congratulates Bari just before disappearing.
Wonrei's fight against Unko Tin Tin
- more specifically when he used his own body to shield Li Yen, Tio, and Megumi from Unko Tin Tin's self destruct.
  - ...and survives.
Tio shielding everyone from Clear Note's attack
In one of the episodes of Zatch Bell, when Zatch was hanging on trying to save Naomi from a big lion, all hope seems lost. Naomi, who was scared at first, looked up at Zatch. When the lion was about to crush her, she jumped very high and started climbing like there was no tomorrow to save Zatch and herself!
Two words (and a warning that happens at the last arc): Brago Punch.
Chapters 250-252 are a collective one for Monmon. When he learns that Kiyomaro is alive and returning to the battlefield, the little monkey finds his courage and devotes himself to protecting Gash no matter what. He fights without a single spell that can inflict injury, and runs rings around both Jedun and Rodeux, using his spells to their fullest effect(and learning two more mid battle), and his ability to sense magic to easily dodge both of them. He nearly wins the fight too, only failing when Elle receives an injury that forces him to save her first at the cost of his book. He does however buy time for Kiyomaro to return, which was his original intention.
A small, overlooked one for filler character Shion Hibiki. In the fifth and final episode featuring her (by which her spellbook has already been burned and her mamodo partner has left her) she's filled with a new resolve, snaps out of her Heroic BSOD, goes all Action Girl and throws herself at the arc's main villain in order to knock the MacGuffin out of his hand. This action ends up breaking said MacGuffin and allowing Zatch to defeat the bad guys. And seeing as said bad guys had screwed with Shion this whole story, it's definitely a case of Laser-Guided Karma.