Laharl gets some of the most brutal moments, such as obliterating the Earth fleet singlehandedly or leveling the Serpahic Sanctum in a fit of righteous rage... and, most particularly, the ending: "IF THERE IS A GOD, MAY HE HEAR MY PLEA! I, LAHARL, HAVE ONE REQUEST: TAKE MY LIFE, IN EXCHANGE FOR FLONNE'S!" Come on, you know you got a little shiver at that.
"Your new name is Mid-boss".
Mid-Boss, incidentally, gets one, too. At the end of Chapter 13, as General Carter is getting away from the heroes and is panicking about how powerful demons are, Mid-Boss shows up and demands to know who Carter was referring to when he said that "he didn't tell me they were this powerful". Unfortunately, he quickly ruins the moment by congratulating himself for being so awesome.
He also has an attack that qualifies: Meteor Impact, where he jumps up into the air so damn high he's able to hijack a passing meteor and sends it towards his opponent while riding on top of it and laughing maniacally.
Flonne's speech against Vulcanus in the final chapter.
Speaking of Batman Gambits, SeraphLamington's gambit is very impressive starting from sending Flonne in an assassination mission while in reality is the beginning of his attempt to rebuild a bridge of peace between Celestia and the Netherworld and even added his treacherous second-in-command Vulcanus's own gambit into the mix with Mid-Boss the only person who knew about this gambit before its reveal. Impressive chain of Unwitting Pawns too, Captain Gordon, Defender Of Earth! -> General Carter -> Vulcanus -> Lamington.
Etna turning into a Reverse Mole against Maderas and revealing she helped Laharl with his bid for overlord so eventually he would reveal himself. And when Maderas tries to order the Prinny Squad to attack them, Etna casually and faux-sweetly tells him she's bought them from under him.