Mage: the Funnel of Frost, which is a fire extinguisher that can strip away beneficial buffs and freezes people if done perfectly (can also be upgraded to cause MORE damage the more buffs stripped), and the Fire Pyre, which is a dangerously large fireworks rocket strapped with four smaller, dangerously powerful firecrackers. Provided the enemies aren't affected by any status effects that change their animation, they run around screaming and panicking. Essentially, it's the 4th grader's version of a nuclear bomb.
Bonus points in how casual the player looks while he's lighting it up, and the oh so satisfying click of the lighter.
Thief: "Execute", which has you leaping into the air and raining kunai down on your target before crushing them with a flail the size of your oversized head, and "Death of a thousand cuts", which summons a squad of ninja to Teleport Spam and cut your target to ribbons.
Jew: you can literally summon the "Plagues of Egypt", which damage foes and give them a variety of nasty status effects.
Stan: what could be the swordsman's final attack? TheOmnislash.
Kenny: he can summon both an army of mice and a unicorn. Whether or not you do them correctly (failure results in Kenny's death), they make clear that they are not cutesy sissies. Especially the enormous horse with the dangerous horn.
At the end of the second day, you have to choose to choose between Cartman or Kyle. Depending of you choice, you will have a duel boss fight against either of them. At the end of the duel, you will have a Fart O War between either of them (Cartman using a fiery fart, while Kyle makes do with a leafblower).
It then turns out that Clyde has manipulated both sides of the fight to take the Stick of Truth for himself and the only thing that stops the heroes from taking him out then and there is that it's late and the kids all need to go home before they're grounded.
The final boss fight with Princess Kenny. When he uses his special abilities (summoning his Nazi zombie unicorn, Nazi zombie rats, Nazi zombie Death, and drink more of the green goo), your party members can prevent him from succeeding with their abilities. Stan would decapitate his Nazi zombie unicorn, Jimmy would play his pipe to lure the Nazi zombie rats away, Butters would transform into Professor Chaos and blast Nazi zombie Death away, and Kyle would kick Ike towards Kenny to stop him from drinking more green goo.
Even after all that, Kenny keeps raising from the dead, thus making Cartman hold Kenny so you can fart in his balls.
"Dragon's Breath" is incredibly powerful with the right equipment, too. Given its multi-hit, armour melting upgrades, it can be used free, with bonus PP, and bring your health back up to full, all in one turn, while destroying the enemies shields, a good chunk of their health, and, if performed perfectly, will set them on fire, causing a DoT. Combine with a Magic attack like Dragonshout to cause even MORE massive damage, and stun them, to boot. It's also really, really funny to watch them scream and panic.
There's nothing quite like clearing out an entire room with nothing but environmental hazards.
While New Kid's parents are having sex in the background, you end up fighting the gnomish warlock. During said fight, there's a quick time event that involves dodging your dad's balls. How does New Kid dodge it? With an uber Badass backflip dodge.
Five Words: BY THE HAMMER OF BUTTERS!
Infiltrating Clyde's Fortress towards the end of the game. Sure, the whole thing is only a treehouse in the guy's backyard, but storming into it with your Badass Army and knocking out everyone in sight with environmental hazards or allowing teammates inside to attack for you makes it impossibly epic.
Meta example: This game blows the idea that video games based on existing franchises will always suck completely out of the water. And then farts on its balls.
Just the amount of time spent on in-jokes and references and making it feel like an interactive episode of South Park is impressive.