Episode 1 when Ryutaro unknowningly runs over Momotaros when he asks for his wish.
How are we introduced to the main character? He managed to get stuck on top of a tree on his bike.
Episode 2 with Momotaro at the receiving end with a slap upside his head by Hana.
For starters, how about Naomi's Gold Gentleman Coffee, which makes Momotaros and Kintaros act like polite, cultured gentlemen, complete with Bishie Sparkle? Oh, and then Momo mimicked Tendou'sCharacter Tic!
Deneb and the watermelon helmet in episode 27. "I made this by biting!"
Back in the original Den-O, we have the hostage episode. Ryotaro is being held hostage, and Airi's reaction is...keep making coffee. When Ozaki asks how she can be so calm in this situation, she realizes he's right, and starts making "Ryo-chan" some special hostage snacks. EVERY man in the Milk Dipper Face Faults.
In that very same episode, Ryutaros is out loose and about to attack the man that is causing the whole situation (Airi is also in the same house where Ryotaro was being held hostage) and the only thing that can stop him? Momotaros forcing him to call Airi. Even better? Just listen to their ringtones.
Plat Form nearly got his, standing on a building ready for battle...until target Imagin fires at him, causing him to lose balance and fall off.
Ryotaro liking the Climax Form, and then imagining it as a dancing sensation. Even better, the Taros apparently saw it.
Momotaros:(shaking his hips in Climax Form) Ore...Fever!
Ryutaros doing a Victory Dance while in Climax Form's Kick Mode, causing Momotaros to wind up in a painful split.
While not-named Kintaros is seemingly about to disappear, Ryutaro suggests Kintaros get inside him to use his power. Momotaros and Urataros spitting his coffeedid not take this well
During the battle near the start of the episode, Ryoutaro informs Momotaros he's going against his battle advice (for when Ryoutaro is in Liner Form) from a few episodes ago, Momotaros tells both Ryoutaro and the Imagin to forget what just happens, and...he starts the fight over again, putting down the Den-Gasher, even saying his catchphrase again!
Look again: Ryuutaros already has a dart stuck in his forehead. It wasn't enough for making him reconsider, apparently.
Deneb possessing Yuuto to make shopping for groceries easier. Or not, as Yuuto's voiceover keeps going 'no shiitake' every chance he gets — so Deneb actually looks away while reaching for shiitake. It doesn't work.
Ryotaro is practicing his swordplay, specifically the art of the Bare-Handed Blade Block. The Taros help him out with this by acting as his sparring partners. Kintaros and Urataros handle this fairly professionally...then it's Momotaros' turn.
Momotaros:(raises practice sword above head as if to strike)RYOTARO!!(points off to the side)What's that? Ryotaro:(looks) *THWACK*
Momotaros: (audibly disappointed) Don't fall for that.
Followed by Ryuutaros not even giving him any warning or getting into position, simply thwacking him the moment he's on camera...and then rightly paying for it when trying the same stuff on the Owner, who gives a hilarious demonstration of Nonchalant Dodge (even blocking one strike with a spoon), disarming and discombobulating Ryuu in a single move , and clocking him on the head hard enough to K.O. him without even looking. One does not simply mess with Owner-san.
A cherry on the moment: Ryuutaro moved to block Owner's strike the way Ryotaro had been practicing, but as bad as Ryo's reaction time was, Ryuu's was even slower.
Followed up by an Awesome Moment when Ryo pulls the block off successfully.
Episode 49, the final one: After Kai's defeat, it looks like the Taroses disappeared with the rest of the Imagins. Well, no: they're just hiding in a nearby trench for surprising Ryotaro. And then they start arguing when they realize they waited for too long and missed the perfect moment for their surprise.
From the Movies and other stuff...
From Climax Deka:
The scene where Ryutaros goes in and starts arresting people who look like they'd be part in an Evil Organization. It goes Up to Eleven because he brought his dance posse with him and they all start beating up these innocent bystanders and handcuff them once they're pummeled to the ground.
Later on, the DenLiner 'precinct' car gets packed with all the buggers he brought in. Among them are male crossdressers...
From Final Countdown:
Den-O Sword Form is fighting a large number of enemies, boasts how he will beat them all, and then slides on a slope. Might be a Throw It In, considering how Momo himself was caught by surprise.
Yuki Hijack Form jumps from a second-story height to ride a horse. Momotaros tells Ryotaro that they should follow suit. Momotaros succeeds, Ryotaro doesn't.
The Imagin Anime episode "The Secret Kamen Rider Room". Especially the part where Ryuutaros finds a magazine of gravure photos of Wakana Matsumoto (Airi) and the other Imagin suddenly look...rather happy. (Except Ura, who immediately confiscates the magazine, and Eve, whom Ura keeps interrupting to avoid accessing info on WHY the staff would have such an item in the office.)
And that ep. where the censoring of inter-franchise references gets thrown out the window, as the Taros meet a potential new member...Ultraman Taro.
The Kamen Rider Den-O arc of Kamen Rider Decade pretty much is the two single most silly episodes in the series. Starts with everyone in the main cast being possessed and ends with Yuusuke mistakenly Rider Kicking one of the heroes and only giving him a krick in the neck as a result.
Momotaros' Final Attack Ride. It's just a souped-up version of Sword Form's own Finishing Move. "Attaching" Kamen Rider Kuuga to the blade, on the other hand...Funnier when you remember that in that form, that Attack means he should be attaching to Kuuga's head. Which says some strange things about the Final Form Rides when your head seems to literally become your ass...
Later on, when Tsukasa decides to try out the Den-O powers for himself, he quickly finds that their Attack Ride cards will cause him to mimic their catchphrases...and nothing else. The ZECT Riders facing him are less than amused, and proceed to bum-rush him...causing the Decade Driver to announce Kintaros' catchphrase as they, indeed, make Tsukasa cry.