There's one cutscene where Mario sets off the detonator after beating Roy Koopa in the fifth castle, only to see the castle not blow up. He goes to take a look at it... and then the castle blows up right in his face.
His expression is priceless.
In the All-Stars version, Luigi gets an even better one- He just looks at the player with a deadpan expression.
In a similar vein, after beating Ludwig von Koopa in the fourth castle, Mario sets off the detonator to blow up the castle... only for the castle to launch into the air like a rocket and impact against a nearby hill... which then wears a bandage where the castle hit for the rest of the game.
"Mama Luigi" provides several:
Luigi: Well, like they say in Brooklyn, early to bed, early to catch the worm...Or is it the bagel?
Yoshi: Mmmmm. Worm's good.
Yoshi becomes attached to Luigi:
Baby Yoshi:[jumps into Luigi's arms and licks his cheek] Mama Luigi!
Luigi: Cool it, caterpillar breath! I'm not youró
Mario: Mama? Mama Luigi?! AHAHAHAHAHA!
After Yoshi eats Mario:
Mario: Now I know how a meatball feels!
Baby Yoshi:[spits Mario out] Ueagh, Yoshi no like meatballs!
When one of the Monty Moles is being pulled up by Yoshi, Cheatsy states sarcastically, "Don't just help him! Stand there!"; the Monty Moles, not understanding sarcasm, do just that, leading to Cheatsy berating them to help said Monty Mole.
Earlier in the episode, when Cheatsy is singing his Villain Song, the Mario brothers and Yoshi get rid of all the Monty Moles when he's not looking, and they end up being the ones singing the final chorus. It's only after the song's over that Cheatsy sees what happened, leading to a Wild Take. Mario's nonchalant smile and wave when Cheatsy sees them is what really sells it.