There are actually several in Abra-Castrophe, but possibly the most crowning one is when Timmy reveals the existence of his godparents, and then Crocker realizes that will set them free and leave him powerless. The look on Crocker's face was, well, a very crowning moment of shock.
Plus Timmy's parents beating the snot out of him.
In the TV movie Wishology where Timmy sacrifices himself to the Darkness to save his friends, family, and even his enemies
Also, in the first part of Wishology where Timmy does not have access to magic, and he has to improvise without it to find a mystical wand. He does very well
Fairly Odd Baby was definitely a great one. The Pixies have Poof and Timmy has a plan; we then cut to them captured with them all having different costumes (Jorgen is in a pink bunny suit, Cosmo's is a taco delivery guy and Wanda is Robin Hood and Timmy's in knight armor). Then Timmy reveals that his plan NEEDED the group to be captured, which escalates into a brilliant plan to save Poof and defeat the Pixies!
Bonus points was Timmy getting Jorgen to dance...in said bunny suit. Jorgen immediately does so.
Mr. Crocker of all characters gets one by single-handedly defeating Miss Doombringer by sending a whole fleet of cars after her while he was reduced to a mere crossing guard after he had broken his arm (due to falling out of a window while spazzing). Even Timmy is impressed.
Information Stupor Highway: Timmy not only keeps Crocker's proof of fairies video from being seen by the world, but also steals the picture of him in his mother's red dress and lets it go viral.
Most Wanted Wish Ever: With some help from Timmy, the winner of the "Fairy Rage in the Cage" is Cosmo!
Chester gets one in Fairy Idol when he snaps on Norm after finding out he'd been manipulating him the entire time. After chewing Norm out, he wishes he'd never found Norm in the first place, happily pressing the Reset Button to undo all the damage he caused.
In "Apartnership", Cosmo standing up to Mama Cosma: "Let's get something straight! I'm not bright! Big words confuse me! I have the attention span of a rodent! But Wanda loves me anyway. She makes me happy and THAT SHOULD BE ENOUGH FOR YOU!"
Another for Cosmo would be in Abra-Castrophe. He's freaking out about Crocker taking over the world and (more importantly) taking Wanda as a magical power source. He knows he's not the brightest, so it looks like he temporarily abandons Timmy in his time of need. But when Cosmo returns, he's ripped and proceeds to pound Crocker to a pulp for kidnapping his wife. A short time later, this awesome moment is slightly dampened by Cosmo ending up in the same magic scepter that Wanda is, but he still deserves credit for a rather impressive attempt to save the day.
The fact is that he did so by going home and watching a video called 30 Second Massive Pecs". Made even more obvious that they canceled out a blast Crocker made.
In "The Fairly Oddlympics" the Anti-Fairies and Pixies knocked out Jorgen in the games, so Timmy rallies the Fairies and coach them in winning the games, even knocking out Jorgen again to prove his point.
A small one, but a good one: near the end of the first Norm episode, Timmy has managed to force Norm into giving him three more wishes (after using up the previously three, which in turn caused Cosmo and Wanda to take his place in his lampnote which Crocker is about to buy. Realizing that he has to avoid falling for tricks again, what does Timmy say?
Even better? The lawyer Timmy gets knows Norm. So, a contract gets written up that fixes everything. Like I said, small but good.
In the episode where the queen of Yugopetamia makes Poof the emperor so that she can get more attention (It Makes Sense in Context), Timmy, Cosmo and Wanda go off to save him. When they get there, they're blocked by an army of guards. What follows is an epic CMOA for the entire Cosma family. Wanda uses the swirl in her hair to toss away the Yugopetamians space ship, Cosmo uses the vacuum he's trapped in (long story) to suck up their weapons leaving them defenseless, and Timmy and Poof top it all off with a tag team wrestling match against the entire army. And they win!Don't mess with the Cosmas!
In "Viral Vidiots" Timmy's mom beating up Crocker and rescuing Timmy.
In "The Switch Glitch", Timmy becomes Vicky's babysitter and ends up making her so unhappy that his godparents are switched over to Vicky, who then forces them to do her bidding. He saves the day when he tricks baby Vicky into reading the line "I'm happy and I don't need my fairy godparents anymore", the special phrase that sets a child's fairy godparents free.
Minor one, but in This Is Your Wish, the studio audience has to vote on whether to leave Cosmo with Timmy or send him home to his mother (Long story). At the end, everyone is about to unanimously vote him home, only for Timmy to change their minds with just a few sentences:
Timmy: Wait! Before you push that button, think about it: Do you really want him back here in Fairy World?
(Cut to Cosmo sucking on his wand like a pacifier. The audience promptly has a Mass "Oh, Crap!" and slams down the "Stay With Timmy" button.)